compositor: make GL_EXT_read_format_bgra optional

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>
6 files changed