avformat/matroskaenc: Avoid seek when writing Cues at the front

avformat/matroskaenc: Avoid seek when writing Cues at the front

When the Cues are written in front of the Cluster, the muxer would seek
to the beginning (to where the Cues ought to be written) and write the
Cues; afterwards it would seek back to the end of the file only to seek
to the beginning once again to update several elements there. This
commit removes the seek to the end.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>