- split packets to store one ASS event line per Matroska block
 - reformat ASS events to Matroska modified format
 - extract display duration for ASS events to store as Matroska block duration


git-svn-id: file:///var/local/repositories/ffmpeg/trunk@15561 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b