app/headless,gpu/gl: make ReadPixels y-flipping backend specific

The Direct3D backend doesn't need y-flipping, so don't do it unconditionally in
package app/headless.

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

app/headless/backend_test.go

@@ -194,7 +194,6 @@ func screenshot(t *testing.T, fbo backend.Framebuffer, size image.Point) *image.
 	if err != nil {
 		t.Fatal(err)
 	}
-	flipImageY(img)
 	if *dumpImages {
 		if err := saveImage(t.Name()+".png", img); err != nil {
 			t.Error(err)