avformat/matroskaenc: Ignore AttachedFiles for track limit

avformat/matroskaenc: Ignore AttachedFiles for track limit

Attachments are streams in FFmpeg, but they are not tracks in Matroska.
Yet they were counted when checking a limit for the number of tracks that
the Matroska muxer imposes. This is unnecessary and has been changed.

Also use unsigned variables for the variables denoting TrackNumbers as
negative TrackNumbers are impossible.

(The Matroska file format actually has practically no limit on the
number of tracks and this is purely what our muxer supports. But even if
this limit were removed/relaxed in the future, it still makes sense to
use small TrackNumbers as this patch does, because greater numbers need
more bytes to encode.)

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