lavf: let av_find_best_stream use bitrate info if available

lavf: let av_find_best_stream use bitrate info if available

I guess the user expects to see the stream with the highest bitrate, not with
the most frames, this is especially useful for multi bitrate streams.

This patch changes av_find_best_stream to select the stream based on a number
of conditions, the first condition has the highest priority, the last condition
has the lowest:

1) Select the stream with the highest FFMIN(5, codec_info_nb_frames) value
2) Select the stream with the highest bitrate
3) Select the stream with the highest codec_info_nb_frames
4) Select the first stream

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>