In mpeg1/2 timestamps are associated with the access unit which
contains the first picture startcode that commences in the PES
packet, instead of the first access unit that commences in the
PES packet. Fix the parser to
handle that properly. This was a very long standing bug ...
The change to the seek regressions is because the mpeg ts muxer
stores too many invalid and randomized timestamps which overflow
the 4 entry buffer we use in the parser.

Originally committed as revision 13643 to svn://svn.ffmpeg.org/ffmpeg/trunk
