Reuse as much code as possible from vaapi-xcb implementation. Most of
the routines can be shared and are placed inside common.{c,h}. The
video output has been moved to modules/video_output/vaapi/, but is
still build from modules/codec/avcodec/Modules.am. This is because
the video output shares the same libva connection as the decoder.
Since this is mandatory for some GPU implementations such as AMD Fusion.