Fix "ost->st->avg_frame_rate = ost->frame_rate" in streamcopy_init()
being reset to input's frame rate a few lines below.
Note that in current code, there are some discrepancies amongst the
muxers. For example, avienc relies on time_base, so it is not affected
by this patch, whereas mxfenc and matroskaenc do use avg_frame_rate,
so this patch fixes -r being honored.
In the updated fate test, the input is (wrongly) probed as 50fps. With
this patch, the correct value (25fps) is successfully forced with -r.
Signed-off-by: Nicolas Gaullier <nicolas.gaullier@cji.paris>
Signed-off-by: Anton Khirnov <anton@khirnov.net>