mpeg2: add codec specific decoder unit.
mpeg2: add codec specific decoder unit.

Introduce new GstVaapiDecoderUnitMpeg2 object, which holds the standard
GstMpegVideoPacket and additional parsed header info. Besides, we now
parse as early as in the _parse() function so that to avoid un-necessary
creation of sub-buffers in _decode() for video packets that are not slices.

ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,CapOM/gstreamer-vaapi,CapOM/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,GStreamer/gstreamer-vaapi,CapOM/gstreamer-vaapi,GStreamer/gstreamer-vaapi,ceyusa/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,gbeauchesne/gstreamer-vaapi,01org/iotg-lin-gfx-gstreamer-vaapi