better and simpler logic for MPEG muxing - fixed rare MPEG muxing PTS...

better and simpler logic for MPEG muxing - fixed rare MPEG muxing PTS generation bug (stuffing is added in such rare cases) - fixed AC3 payload size generation - generate correct AC3 frame header (need spec checking)

Originally committed as revision 2621 to svn://svn.ffmpeg.org/ffmpeg/trunk