Some GL implementations do not provide GL_EXT_read_format_bgra
extension.
Set a glReadPixels format based on whether the extensions is supported
or not, and use that format in all backends.
Add RGBA->BGRA swapping copy to screenshooter to keep the shm buffer
data format as BGRA.
Signed-off-by: Pekka Paalanen <ppaalanen@gmail.com>