decpgssub: make sure all packets get marked as subtitle frames


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6476 b64f7644-9d1e-0410-96f1-a4d463321fa5
libav: fix writing zero mkv blockduration element for subtitles

Zero means non-keyframe according to mkv spec and causes errors in mkvalidator


git-svn-id: svn://svn.handbrake.fr/HandBrake/trunk@6477 b64f7644-9d1e-0410-96f1-a4d463321fa5