Add vaapipostproc element for video postprocessing. So far, only basic
bob deinterlacing is implemented. Interlaced mode is automatically
detected based on sink caps ("interlaced" field).
This new interface allows for upstream and downstream display sharing
that works in both static and dynamic pipelines.
Signed-off-by: Gwenole Beauchesne <gwenole.beauchesne@intel.com>