AVHWAccel.end_frame can run on a worker thread. The assumption of the
frame threading code is that the worker thread will change the AVFrame
image data, not the AVFrame fields. So the AVFrame fields are not synced
back to the main thread. But this breaks videotoolbox due to its special
requirements (everything else is fine). It actually wants to update
AVFrame fields.

The actual videotoolbox frame is now stored in the dummy AVBufferRef, so
it mimics what happens in non-videotoolbox cases. (Changing the
AVBufferRef contents is a bit like changing the image data.) The
post_process callback copies that reference to the proper AVFrame field.

Based on a patch by wm4.
Signed-off-by: Aman Gupta <aman@tmm1.net>