Implement placeholder assets

* Placeholder textures
* Placeholder meshes
* Placeholder material

This PR is the first step towards implementing godotengine/godot-proposals#2756
It adds an asset type that uses no resources, which can be used to replace the existing ones on export for using on the upcoming server export.

doc/classes/PlaceholderCubemap.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderCubemap" inherits="PlaceholderTextureLayered" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+</class>

doc/classes/PlaceholderCubemapArray.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderCubemapArray" inherits="PlaceholderTextureLayered" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+</class>

doc/classes/PlaceholderMaterial.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderMaterial" inherits="Material" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+</class>

doc/classes/PlaceholderMesh.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderMesh" inherits="Mesh" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="aabb" type="AABB" setter="set_aabb" getter="get_aabb" default="AABB(0, 0, 0, 0, 0, 0)">
+		</member>
+	</members>
+</class>

doc/classes/PlaceholderTexture2D.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderTexture2D" inherits="Texture2D" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="size" type="Vector2" setter="set_size" getter="get_size" default="Vector2(1, 1)">
+		</member>
+	</members>
+</class>

doc/classes/PlaceholderTexture2DArray.xml

@@ -0,0 +1,9 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderTexture2DArray" inherits="PlaceholderTextureLayered" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+</class>

doc/classes/PlaceholderTexture3D.xml

@@ -0,0 +1,13 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderTexture3D" inherits="Texture3D" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="size" type="Vector3i" setter="set_size" getter="get_size" default="Vector3i(1, 1, 1)">
+		</member>
+	</members>
+</class>

doc/classes/PlaceholderTextureLayered.xml

@@ -0,0 +1,15 @@
+<?xml version="1.0" encoding="UTF-8" ?>
+<class name="PlaceholderTextureLayered" inherits="TextureLayered" version="4.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:noNamespaceSchemaLocation="../class.xsd">
+	<brief_description>
+	</brief_description>
+	<description>
+	</description>
+	<tutorials>
+	</tutorials>
+	<members>
+		<member name="layers" type="int" setter="set_layers" getter="get_layers" default="1">
+		</member>
+		<member name="size" type="Vector2i" setter="set_size" getter="get_size" default="Vector2i(1, 1)">
+		</member>
+	</members>
+</class>

scene/register_scene_types.cpp

@@ -752,6 +752,7 @@ void register_scene_types() {
 
 	GDREGISTER_VIRTUAL_CLASS(Mesh);
 	GDREGISTER_CLASS(ArrayMesh);
+	GDREGISTER_CLASS(PlaceholderMesh);
 	GDREGISTER_CLASS(ImmediateMesh);
 	GDREGISTER_CLASS(MultiMesh);
 	GDREGISTER_CLASS(SurfaceTool);
@@ -773,6 +774,7 @@ void register_scene_types() {
 	GDREGISTER_ABSTRACT_CLASS(BaseMaterial3D);
 	GDREGISTER_CLASS(StandardMaterial3D);
 	GDREGISTER_CLASS(ORMMaterial3D);
+	GDREGISTER_CLASS(PlaceholderMaterial);
 	SceneTree::add_idle_callback(BaseMaterial3D::flush_changes);
 	BaseMaterial3D::init_shaders();
 
@@ -837,6 +839,12 @@ void register_scene_types() {
 	GDREGISTER_CLASS(CompressedCubemap);
 	GDREGISTER_CLASS(CompressedCubemapArray);
 	GDREGISTER_CLASS(CompressedTexture2DArray);
+	GDREGISTER_CLASS(PlaceholderTexture2D);
+	GDREGISTER_CLASS(PlaceholderTexture3D);
+	GDREGISTER_ABSTRACT_CLASS(PlaceholderTextureLayered);
+	GDREGISTER_CLASS(PlaceholderTexture2DArray);
+	GDREGISTER_CLASS(PlaceholderCubemap);
+	GDREGISTER_CLASS(PlaceholderCubemapArray);
 
 	GDREGISTER_CLASS(Animation);
 	GDREGISTER_CLASS(AnimationLibrary);

scene/resources/material.cpp

@@ -3010,4 +3010,7 @@ bool StandardMaterial3D::_set(const StringName &p_name, const Variant &p_value)
 
 	return false;
 }
+
 #endif // DISABLE_DEPRECATED
+
+///////////////////////

scene/resources/material.h

@@ -797,6 +797,13 @@ class ORMMaterial3D : public BaseMaterial3D {
 			BaseMaterial3D(true) {}
 };
 
+class PlaceholderMaterial : public Material {
+	GDCLASS(PlaceholderMaterial, Material)
+public:
+	virtual RID get_shader_rid() const override { return RID(); }
+	virtual Shader::Mode get_shader_mode() const override { return Shader::MODE_CANVAS_ITEM; }
+};
+
 //////////////////////
 
 #endif

scene/resources/mesh.cpp

@@ -2092,3 +2092,17 @@ ArrayMesh::~ArrayMesh() {
 		RenderingServer::get_singleton()->free(mesh);
 	}
 }
+///////////////
+
+void PlaceholderMesh::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_aabb", "aabb"), &PlaceholderMesh::set_aabb);
+	ADD_PROPERTY(PropertyInfo(Variant::AABB, "aabb", PROPERTY_HINT_NONE, ""), "set_aabb", "get_aabb");
+}
+
+PlaceholderMesh::PlaceholderMesh() {
+	rid = RS::get_singleton()->mesh_create();
+}
+
+PlaceholderMesh::~PlaceholderMesh() {
+	RS::get_singleton()->free(rid);
+}

scene/resources/mesh.h

@@ -328,4 +328,38 @@ VARIANT_ENUM_CAST(Mesh::ArrayCustomFormat);
 VARIANT_ENUM_CAST(Mesh::PrimitiveType);
 VARIANT_ENUM_CAST(Mesh::BlendShapeMode);
 
+class PlaceholderMesh : public Mesh {
+	GDCLASS(PlaceholderMesh, Mesh);
+
+	RID rid;
+	AABB aabb;
+
+protected:
+	static void _bind_methods();
+
+public:
+	virtual int get_surface_count() const override { return 0; }
+	virtual int surface_get_array_len(int p_idx) const override { return 0; }
+	virtual int surface_get_array_index_len(int p_idx) const override { return 0; }
+	virtual Array surface_get_arrays(int p_surface) const override { return Array(); }
+	virtual Array surface_get_blend_shape_arrays(int p_surface) const override { return Array(); }
+	virtual Dictionary surface_get_lods(int p_surface) const override { return Dictionary(); }
+	virtual uint32_t surface_get_format(int p_idx) const override { return 0; }
+	virtual PrimitiveType surface_get_primitive_type(int p_idx) const override { return PRIMITIVE_TRIANGLES; }
+	virtual void surface_set_material(int p_idx, const Ref<Material> &p_material) override {}
+	virtual Ref<Material> surface_get_material(int p_idx) const override { return Ref<Material>(); }
+	virtual int get_blend_shape_count() const override { return 0; }
+	virtual StringName get_blend_shape_name(int p_index) const override { return StringName(); }
+	virtual void set_blend_shape_name(int p_index, const StringName &p_name) override {}
+	virtual RID get_rid() const override { return rid; }
+	virtual AABB get_aabb() const override { return aabb; }
+	void set_aabb(const AABB &p_aabb) { aabb = p_aabb; }
+
+	virtual int get_builtin_bind_pose_count() const override { return 0; }
+	virtual Transform3D get_builtin_bind_pose(int p_index) const override { return Transform3D(); }
+
+	PlaceholderMesh();
+	~PlaceholderMesh();
+};
+
 #endif

scene/resources/texture.cpp

@@ -3395,3 +3395,148 @@ CameraTexture::~CameraTexture() {
 		RenderingServer::get_singleton()->free(_texture);
 	}
 }
+
+///////////////////////////
+
+void PlaceholderTexture2D::set_size(Size2 p_size) {
+	size = p_size;
+}
+
+int PlaceholderTexture2D::get_width() const {
+	return size.width;
+}
+
+int PlaceholderTexture2D::get_height() const {
+	return size.height;
+}
+
+bool PlaceholderTexture2D::has_alpha() const {
+	return false;
+}
+
+Ref<Image> PlaceholderTexture2D::get_image() const {
+	return Ref<Image>();
+}
+
+RID PlaceholderTexture2D::get_rid() const {
+	return rid;
+}
+
+void PlaceholderTexture2D::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_size", "size"), &PlaceholderTexture2D::set_size);
+
+	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2I, "size"), "set_size", "get_size");
+}
+
+PlaceholderTexture2D::PlaceholderTexture2D() {
+	rid = RS::get_singleton()->texture_2d_placeholder_create();
+}
+
+PlaceholderTexture2D::~PlaceholderTexture2D() {
+	RS::get_singleton()->free(rid);
+}
+
+///////////////////////////////////////////////
+
+void PlaceholderTexture3D::set_size(const Vector3i &p_size) {
+	size = p_size;
+}
+
+Vector3i PlaceholderTexture3D::get_size() const {
+	return size;
+}
+
+Image::Format PlaceholderTexture3D::get_format() const {
+	return Image::FORMAT_RGB8;
+}
+
+int PlaceholderTexture3D::get_width() const {
+	return size.x;
+}
+
+int PlaceholderTexture3D::get_height() const {
+	return size.y;
+}
+
+int PlaceholderTexture3D::get_depth() const {
+	return size.z;
+}
+
+bool PlaceholderTexture3D::has_mipmaps() const {
+	return false;
+}
+
+Vector<Ref<Image>> PlaceholderTexture3D::get_data() const {
+	return Vector<Ref<Image>>();
+}
+
+void PlaceholderTexture3D::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_size", "size"), &PlaceholderTexture3D::set_size);
+	ClassDB::bind_method(D_METHOD("get_size"), &PlaceholderTexture3D::get_size);
+	ADD_PROPERTY(PropertyInfo(Variant::VECTOR3I, "size"), "set_size", "get_size");
+}
+
+PlaceholderTexture3D::PlaceholderTexture3D() {
+	rid = RS::get_singleton()->texture_3d_placeholder_create();
+}
+PlaceholderTexture3D::~PlaceholderTexture3D() {
+	RS::get_singleton()->free(rid);
+}
+
+/////////////////////////////////////////////////
+
+void PlaceholderTextureLayered::set_size(const Size2i &p_size) {
+	size = p_size;
+}
+
+Size2i PlaceholderTextureLayered::get_size() const {
+	return size;
+}
+
+void PlaceholderTextureLayered::set_layers(int p_layers) {
+	layers = p_layers;
+}
+
+Image::Format PlaceholderTextureLayered::get_format() const {
+	return Image::FORMAT_RGB8;
+}
+
+TextureLayered::LayeredType PlaceholderTextureLayered::get_layered_type() const {
+	return layered_type;
+}
+
+int PlaceholderTextureLayered::get_width() const {
+	return size.x;
+}
+
+int PlaceholderTextureLayered::get_height() const {
+	return size.y;
+}
+
+int PlaceholderTextureLayered::get_layers() const {
+	return layers;
+}
+
+bool PlaceholderTextureLayered::has_mipmaps() const {
+	return false;
+}
+
+Ref<Image> PlaceholderTextureLayered::get_layer_data(int p_layer) const {
+	return Ref<Image>();
+}
+
+void PlaceholderTextureLayered::_bind_methods() {
+	ClassDB::bind_method(D_METHOD("set_size", "size"), &PlaceholderTextureLayered::set_size);
+	ClassDB::bind_method(D_METHOD("get_size"), &PlaceholderTextureLayered::get_size);
+	ClassDB::bind_method(D_METHOD("set_layers", "layers"), &PlaceholderTextureLayered::set_layers);
+	ADD_PROPERTY(PropertyInfo(Variant::VECTOR2I, "size"), "set_size", "get_size");
+	ADD_PROPERTY(PropertyInfo(Variant::INT, "layers", PROPERTY_HINT_RANGE, "1,4096"), "set_layers", "get_layers");
+}
+
+PlaceholderTextureLayered::PlaceholderTextureLayered(LayeredType p_type) {
+	layered_type = p_type;
+	rid = RS::get_singleton()->texture_2d_layered_placeholder_create(RS::TextureLayeredType(layered_type));
+}
+PlaceholderTextureLayered::~PlaceholderTextureLayered() {
+	RS::get_singleton()->free(rid);
+}