fix sub timing in mkv files with video delay

Some matroska files have audio start at timestamp 0 and video later.
In this case mkvtoolnix seems to use the first block of the first
cluster to the audio track (I would assume this is only an
implementation detail and not really from the matroska specs. And also
could happen in other cases without the video being delayed, but that's
not the point). Aegisub used to read this first block and use its
timestamp as the starting point of the video track.

With this commit, Aegisub tries to read all the blocks until it can read
the first timestamp of the video track and use it for the subtitles'
timestamps. Audio tracks don't seem to be impacted by these changes.