avformat/mov: use rfps_add_frame() only for video

it makes no sense for other streams

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: reject poorly fitting rfps values earlier

This avoids collecting statistics for rfps values that very likely
will get rejected later.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>