The Matroska file format has practically no limit on the number of
tracks (the current limit is 2^56 - 1); yet because they are encoded in
a variable length format in (Simple)Blocks this muxer has simply imposed
a limit on the number of tracks in order to ensure that they can always
be written on one byte in order to simplify the muxing process.

This commit removes said limit.

Also, zero is an invalid TrackNumber, so disallow this value in the
dash_track_number option.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>