The earlier code relied on the length of clusters always being coded on
eight bytes as was the behaviour of libavformat's Matroska muxer until
recently. But given that our own Matroska muxer now (and mkvmerge from
time immemorial) creates files that don't conform to this assumption,
it is high time to get rid of this assumption.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>