tsdemux: implement proper seeking for h264 streams.

Co-Authored by: Thibault Saunier <tsaunier@gnome.org>

From a high level perspective, the new process for seeking h264
streams is as follows:

1) Rewind the stream until we find the first I-slice of a frame,
   and mark its offset in the stream.
2) Rewind the stream until we find SPS and PPS informations,
   to make sure the subsequent parser is up to date.
3) Accumulate optionnal SEI NAL units on the way.
4) Push the SPS, PPS and SEI units before the new keyframe.