gp/internal/opengl: support sRGB emulation for embedded content

Programs such as gio-example/glfw rely on Gio drawing blending with
the framebuffer background. This change makes it so when sRGB emulation
is active.

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

gpu/gpu.go

@@ -426,7 +426,7 @@ func (g *gpu) Collect(viewport image.Point, frameOps *op.Ops) {
 
 func (g *gpu) Frame() error {
 	viewport := g.renderer.blitter.viewport
-	defFBO := g.ctx.BeginFrame(viewport)
+	defFBO := g.ctx.BeginFrame(g.drawOps.clear, viewport)
 	defer g.ctx.EndFrame()
 	for _, img := range g.drawOps.imageOps {
 		expandPathOp(img.path, img.clip)