If the API user doesn't set avg_frame_rate, matroskaenc will write the
current timebase as "default duration" for the video track. This makes
no sense, because the "default duration" implies the framerate of the
video. Since the timebase is forced to 1/1000, this will make the
resulting file claim 1000fps.

Drop it and don't write the element. It's optional, so it's better not
to write it if the framerate is unknown.

Strangely does not require FATE changes.