This makes the logic a little bit easier to follow; this is *either*
about ES2 compatibility *or* about gles. GL_RGB565 was added already in
OpenGL ES 1.0.
Signed-off-by: Erik Faye-Lund <erik.faye-lund@collabora.com> Reviewed-by: Marek Olšák <marek.olsak@amd.com>