matroskadec: fix ASS subtitle track packets before emitting them

Matroska does some butchering when storing the ASS lines. The start and end
time are removed (because they are duplicated in the container).
The matroska_fix_ass_packet() function simply restore those start and end
time in ASS lines to ensure our ASS packets comply with the ASS spec.

Originally committed as revision 15211 to svn://svn.ffmpeg.org/ffmpeg/trunk