mkv_cuepoint (the structure used to store the index entries in the
Matroska muxer) currently contains fields for both the index of the
packet's stream in the AVFormatContext.streams array and for the
Matroska TrackNumber; correspondingly, mkv_add_cuepoint() has parameters
for both. But these two numbers can't be chosen independently, so get
rid of the TrackNumber.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>