Move libavcodec managed objects from the public struct vaapi_context
to a new privately owned FFVAContext. This is done so that to clean up
and streamline the public structure, but also to prepare for new codec
support, thus requiring new internal data to be added in there.

The AVCodecContext.hwaccel_context, that holds the public vaapi_context,
shall no longer be accessed from within vaapi_*.c codec support files.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>