gpu/internal/opengl,app/internal/wm: generalize desktop OpenGL support

This changes moves the macOS specific setup for desktop OpenGL to the
portable opengl package. The opengl package already takes care of the
desktop OpenGL setup for sRGB framebuffers, and by moving the code we
avoid calling the wrong OpenGL functions in case both OpenGL.framework
and ANGLE libGLESv2.dylib is linked into the program.

Remove the interface casting expressions for gl.Functions; it wasn't
worth the trouble to keep updated.

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/internal/opengl/opengl.go

@@ -36,6 +36,10 @@ type Backend struct {
 	sRGBFBO     *srgb.FBO
 	enabledSRGB bool
 	defFBO      gl.Framebuffer
+
+	// defVertArray is bound during a frame. We don't need it, but
+	// core desktop OpenGL profile 3.3 requires some array bound.
+	defVertArray gl.VertexArray
 }
 
 // State tracking.
@@ -205,6 +209,10 @@ func (b *Backend) BeginFrame(clear bool, viewport image.Point) driver.Framebuffe
 		if b.enabledSRGB {
 			b.funcs.Enable(gl.FRAMEBUFFER_SRGB)
 		}
+		if !b.defVertArray.Valid() {
+			b.defVertArray = b.funcs.CreateVertexArray()
+		}
+		b.funcs.BindVertexArray(b.defVertArray)
 	}
 	b.funcs.BindFramebuffer(gl.FRAMEBUFFER, renderFBO)
 	if b.sRGBFBO != nil && !clear {
@@ -230,6 +238,8 @@ func (b *Backend) EndFrame() {
 	if b.enabledSRGB {
 		b.funcs.Disable(gl.FRAMEBUFFER_SRGB)
 	}
+	// For single-buffered framebuffers such as on macOS.
+	b.funcs.Flush()
 }
 
 func (b *Backend) Caps() driver.Caps {
@@ -368,6 +378,9 @@ func (b *Backend) Release() {
 	if b.sRGBFBO != nil {
 		b.sRGBFBO.Release()
 	}
+	if b.defVertArray.Valid() {
+		b.funcs.DeleteVertexArray(b.defVertArray)
+	}
 	*b = Backend{}
 }