gpu: don't automatically clear screen before rendering

Gio UI may be overlaid on top of custom graphics such as in the glfw example.
That will only work if Gio doesn't clear the screen (to white).

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

internal/rendertest/util_test.go

@@ -161,7 +161,7 @@ func verifyRef(t *testing.T, img *image.RGBA, frame int) (ok bool) {
 	}
 	ref, ok := r.(*image.RGBA)
 	if !ok {
-		t.Error("ref image note RGBA")
+		t.Errorf("image is a %T, expected *image.RGBA", r)
 		return
 	}
 	if len(ref.Pix) != len(img.Pix) {