- Added force_pts to av_write_packet() to be able to force PTS, this helps

(and fix) stream copying. By now force_pts it's just honoured by the MPEG
muxer. ASF could honour this also, but it should be fixed to use Tickers
first.
- MPEG audio decoder exports it's frame size in bytes.
- Hope this fix the floating point exception found in ffserver.

Originally committed as revision 382 to svn://svn.ffmpeg.org/ffmpeg/trunk