Up until now, mkv_write_track() received the index of the stream whose
header data it is about to write as parameter; this index has until
recently been explicitly used to generate both TrackNumber and TrackUID.
But this is no longer so and as there is no reason why the function
for writing a single TrackEntry should even know the index of the
TrackEntry it is about to write, said index is replaced in the list of
function parameters by the corresponding AVStream and mkv_track.
Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>