This fixes decoding of H264 video samples with SPS and PPS changes.

See for example https://s3.amazonaws.com/tmm1/videotoolbox/spschange.ts,
which previously stalled the decoder and failed to produce any new frames
after the SPS change.

Also see https://s3.amazonaws.com/tmm1/videotoolbox/ppschange.ts, which
uses multiple PPS and would previously cause VT decode failures.

If the VideoToolbox session needs to be restarted, and
videotoolbox_start() fails for some reason (for instance, if the video
is interlaced and the decoder is running on iOS), avcodec will return
AVERROR_EXTERNAL. This can be used by the API user to switch to another
decoder.
Signed-off-by: Aman Gupta <aman@tmm1.net>