avcodec: fix wording in AVSubtitleRect doxy
avformat/utils: use r_frame_rate only in the demuxer for ff_compute_frame_duration()

This is primarly to maintain current behavior when r_frame_rate is set for muxers
and could be reverted if it has some advantage

Fixes Ticket3629 part1

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
ffmpeg: forward r_frame_rate to the muxer in case of stream copy

Fixes Ticket3629 part2

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/nutenc: store the actual r_frame_rate in the corresponding field if available

Fixes Ticket 3629 part3

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