LibWeb: Implement WebGL2's readPixels with a byte offset argument

We pass it in as a `void*` since that's what this API expects when you
have bound the GL_PIXEL_PACK_BUFFER buffer.

Author: gmta

Libraries/LibWeb/WebGL/WebGL2RenderingContextImpl.cpp

@@ -1496,6 +1496,19 @@ void WebGL2RenderingContextImpl::read_pixels(WebIDL::Long x, WebIDL::Long y, Web
     glReadPixelsRobustANGLE(x, y, width, height, format, type, span.size(), nullptr, nullptr, nullptr, span.data());
 }
 
+void WebGL2RenderingContextImpl::read_pixels(WebIDL::Long x, WebIDL::Long y, WebIDL::Long width, WebIDL::Long height,
+    WebIDL::UnsignedLong format, WebIDL::UnsignedLong type, WebIDL::LongLong offset)
+{
+    m_context->make_current();
+
+    if (!m_pixel_pack_buffer_binding) {
+        set_error(GL_INVALID_OPERATION);
+        return;
+    }
+
+    glReadPixelsRobustANGLE(x, y, width, height, format, type, 0, nullptr, nullptr, nullptr, reinterpret_cast<void*>(offset));
+}
+
 void WebGL2RenderingContextImpl::active_texture(WebIDL::UnsignedLong texture)
 {
     m_context->make_current();

Libraries/LibWeb/WebGL/WebGL2RenderingContextImpl.h

@@ -132,6 +132,7 @@ class WebGL2RenderingContextImpl : public WebGLRenderingContextBase {
     void uniform_matrix3fv(GC::Root<WebGLUniformLocation> location, bool transpose, Float32List data, WebIDL::UnsignedLongLong src_offset, WebIDL::UnsignedLong src_length);
     void uniform_matrix4fv(GC::Root<WebGLUniformLocation> location, bool transpose, Float32List data, WebIDL::UnsignedLongLong src_offset, WebIDL::UnsignedLong src_length);
     void read_pixels(WebIDL::Long x, WebIDL::Long y, WebIDL::Long width, WebIDL::Long height, WebIDL::UnsignedLong format, WebIDL::UnsignedLong type, GC::Root<WebIDL::ArrayBufferView> pixels);
+    void read_pixels(WebIDL::Long x, WebIDL::Long y, WebIDL::Long width, WebIDL::Long height, WebIDL::UnsignedLong format, WebIDL::UnsignedLong type, WebIDL::LongLong offset);
     void active_texture(WebIDL::UnsignedLong texture);
     void attach_shader(GC::Root<WebGLProgram> program, GC::Root<WebGLShader> shader);
     void bind_attrib_location(GC::Root<WebGLProgram> program, WebIDL::UnsignedLong index, String name);

Libraries/LibWeb/WebGL/WebGL2RenderingContextOverloads.idl

@@ -62,6 +62,6 @@ interface mixin WebGL2RenderingContextOverloads {
     // WebGL1:
     undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, [AllowShared] ArrayBufferView? pixels);
     // WebGL2:
-    [FIXME] undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);
+    undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, GLintptr offset);
     [FIXME] undefined readPixels(GLint x, GLint y, GLsizei width, GLsizei height, GLenum format, GLenum type, [AllowShared] ArrayBufferView dstData, unsigned long long dstOffset);
 };