lavf: make overlay_qsv work based on framesync

lavf: make overlay_qsv work based on framesync

The existing version which was cherry-picked from Libav does not work
with FFmpeg framework, because ff_request_frame() was totally
different between Libav (recursive) and FFmpeg (non-recursive).
The existing overlay_qsv implementation depends on the recursive version
of ff_request_frame to trigger immediate call to request_frame() on input pad.
But this has been removed in FFmpeg since "lavfi: make request_frame() non-recursive."
Now that we have handy framesync support in FFmpeg, so I make it work
based on framesync. Some other fixing which is also needed to make
overlay_qsv work are put in a separate patch.

Signed-off-by: Ruiling Song <ruiling.song@intel.com>