playback broken for non-IDR key frames

[mjneil]

key frames can be seekable without being an IDR frame if they include an SEI message with a recovery point. We should be able to detect the presence of this message and mark the frame as a key frame so the mp4 sample.flags.dependsOn is set correctly

http://forum.doom9.org/showthread.php?t=139957 https://bugzilla.mozilla.org/show_bug.cgi?id=1347518

hls.js is doing this here by detecting sps in the same packet and using the non-idr nal slicetype https://github.com/video-dev/hls.js/blob/master/src/demux/tsdemuxer.js#L596