avcodec: refactor frame_rate probing into own function
if codec has list of supported framerates, select highest one unless user has provided one. If we end up using CLOCK_FREQ in time_base, it should be quite ok in most cases. Tested with ts/mp4/webm and mp2v/mp4v/VP80. For some cases it can select higher fps in mpeg-codecs (60fps) in case of input fps isn't yet known in that point or is fubared. Also add special case for MP4V as it doesn't like CLOCK_FREQ in time_base/frame_rates.
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