avformat/utils: provide more IO statistics in debug output at start and end of av_find_streaminfo()

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/utils: factor rfps calculation out

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avformat/mov: calculate rfps from the first 100 frames of the index

This reduces the amount of frames that need to be demuxed in av_find_stream_info()

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