In mpeg1/2 timestamps (against all logic) are associated with

In mpeg1/2 timestamps (against all logic) are associated with
picture start codes and not with access units. 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.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@13643 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b