gpu/internal/opengl: restore BeginFrame state in EndFrame

To ease the integration with foreign OpenGL contexts, carefully save the
context state before rendering a frame and restore it afterwards. Gio
rendering can then be mixed with OpenGL code that expects exclusive
control over context state.

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

gpu/gpu.go

@@ -365,6 +365,8 @@ func New(api API) (GPU, error) {
 	if err != nil {
 		return nil, err
 	}
+	d.BeginFrame(false, image.Point{})
+	defer d.EndFrame()
 	forceCompute := os.Getenv("GIORENDERER") == "forcecompute"
 	feats := d.Caps().Features
 	switch {