avformat/matroskaenc: Avoid allocation for Cues

avformat/matroskaenc: Avoid allocation for Cues

Up until now, the Matroska muxer would allocate a structure containing
three members: The segment offset, a pointer to an array containing Cue
(index) entries and a counter for said array. It is unnecessary to
allocate it separately and it is unnecessary to contain the segment
offset in said structure, as it duplicates another field contained in
the MatroskaMuxContext. This commit implements the corresponding
changes.

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