lavf: deprecate r_frame_rate.
According to its description, it is supposed to be the LCM of all the frame durations. The usability of such a thing is vanishingly small, especially since we cannot determine it with any amount of reliability. Therefore get rid of it after the next bump. Replace it with the average framerate where it makes sense. FATE results for the wtv and xmv demux tests change. In the wtv case this is caused by the file being corrupted (or possibly badly cut) and containing invalid timestamps. This results in lavf estimating the framerate wrong and making up wrong frame durations. In the xmv case the file contains pts jumps, so again the estimated framerate is far from anything sane and lavf again makes up different frame durations. In some other tests lavf starts making up frame durations from different frame.
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