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/headless/driver_test.go

@@ -174,7 +174,7 @@ func newDriver(t *testing.T) driver.Device {
 	if err != nil {
 		t.Fatal(err)
 	}
-	b.BeginFrame(image.Pt(1, 1))
+	b.BeginFrame(true, image.Pt(1, 1))
 	t.Cleanup(func() {
 		b.EndFrame()
 		ctx.ReleaseCurrent()