(Also modifies obs-ffmpeg to handle empty frames on EOF)
Previously the demuxer could hit EOF before the decoder threads are
finished, resulting in truncated output. In the worse case scenario the
demuxer could read small files before ff_decoder_refresh even has a chance
to start the clocks, resulting in no output at all.
Fixes warnings with deprecated packet functions (av_free_packet and
av_dup packet, which were replaced by av_packet_unref and av_packet_ref
respectively)
This attaches clocks to packets and frames and defers
the start time until that particular frame is presented.
Any packets/frames in the future with the same clock
will reference that start time.
This fixes issues when there are multiple start times
in a large buffer (looped video/images/audio) and different
frames need different reference clocks to present correctly.
