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/internal/driver/driver.go

@@ -12,7 +12,7 @@ import (
 // APIs such as OpenGL, Direct3D useful for rendering Gio
 // operations.
 type Device interface {
-	BeginFrame(viewport image.Point) Framebuffer
+	BeginFrame(clear bool, viewport image.Point) Framebuffer
 	EndFrame()
 	Caps() Caps
 	NewTimer() Timer