feat: select the OpenGL version to use.

wraps the go-gl/gl lib in a internal package with build constrains to
let the user choose which go-gl/gl version he desire.

support 3.3(default) and none.

When the build tag is set to none, no dependency to go-gl/gl is
required, but texture plugins will not be supported.

Author: pchampio

application.go

@@ -208,7 +208,7 @@ func (a *Application) Run() error {
 		return true
 	}
 	a.engine.GLProcResolver = func(procName string) unsafe.Pointer {
-		return opengl.GetProcAddress(procName)
+		return glfw.GetProcAddress(procName)
 	}
 	a.engine.GLExternalTextureFrameCallback = texturer.handleExternalTexture
 

internal/opengl/doc.go

@@ -5,5 +5,5 @@
 // building go-flutter.
 // default is v3.3
 //
-// Golang build constraints are used to do the version selection.
+// Golang build constraints are used for version selection.
 package opengl

internal/opengl/version.go renamed to internal/opengl/opengl.go

@@ -1,6 +1,11 @@
-// The default version of OpenGL used by go-flutter.
+// +build !openglnone
+
 package opengl
 
+// The default version (3.3) of OpenGL used by go-flutter.
+// If you want to support other version, copy/pase this file, change the import
+// statement, add builds constraints and open a PR.
+
 import (
 	"unsafe"
 
@@ -9,8 +14,10 @@ import (
 )
 
 // const exposed to go-flutter
-const TEXTURE_2D = gl.TEXTURE_2D
-const RGBA8 = gl.RGBA8
+const (
+	TEXTURE2D = gl.TEXTURE_2D
+	RGBA8     = gl.RGBA8
+)
 
 // Init opengl
 func Init() error {
@@ -67,11 +74,4 @@ func GLFWWindowHint() {
 	glfw.WindowHint(glfw.ContextVersionMinor, 3)
 	glfw.WindowHint(glfw.OpenGLProfile, glfw.OpenGLCoreProfile)
 	glfw.WindowHint(glfw.OpenGLForwardCompatible, glfw.True)
-
-}
-
-// GetProcAddress returns the address of the specified OpenGL or OpenGL ES core
-// or extension function, if it is supported by the current context.
-func GetProcAddress(namea string) unsafe.Pointer {
-	return gl.GlowGetProcAddress(namea)
 }

internal/opengl/opengl_none.go

@@ -0,0 +1,42 @@
+// +build openglnone
+
+package opengl
+
+// Don't link go-flutter against OpenGL, less-dependency at the cost of not
+// supporting external texture (eg: video_plugin)
+//
+// compile go-flutter with: hover build|run --opengl=none
+
+import (
+	"unsafe"
+)
+
+// const exposed to go-flutter
+const (
+	TEXTURE2D = 0
+	RGBA8     = 0
+)
+
+// Init opengl
+func Init() error { return nil }
+
+// DeleteTextures deletes named textures
+func DeleteTextures(n int32, textures *uint32) {}
+
+// CreateTexture creates a texture for go-flutter uses
+func CreateTexture(texture *uint32) {}
+
+// BindTexture binds a named texture to a texturing target
+func BindTexture(texture uint32) {}
+
+// Ptr takes a slice or pointer (to a singular scalar value or the first
+// element of an array or slice) and returns its GL-compatible address.
+func Ptr(data interface{}) unsafe.Pointer { return nil }
+
+// TexImage2D specifies a two-dimensional texture image
+func TexImage2D(width, height int32, pixels unsafe.Pointer) {
+	panic("go-flutter: go-flutter wasn't compiled with support for external texture plugin.")
+}
+
+// GLFWWindowHint sets hints for the next call to CreateWindow.
+func GLFWWindowHint() {}

texture-registry.go

@@ -130,7 +130,7 @@ func (t *TextureRegistry) handleExternalTexture(textureID int64,
 	)
 
 	return &embedder.FlutterOpenGLTexture{
-		Target: opengl.TEXTURE_2D,
+		Target: opengl.TEXTURE2D,
 		Name:   registration.texture,
 		Format: opengl.RGBA8,
 	}