Improve VFR support

[j0sh]

Improve VFR support ( 55a5d80 )

Manually calculate the duration of each frame and set
the PTS to that before submitting to the filtergraph.

This allows us to better support variable frame rates,
and is also better aligned with how ffmpeg does it.

This may change the number of frames output by the FPS
filter by +/- 1 frame. These aren't issues in themselves
but breaks a lot of test cases which will need to be updated.

Update testcases for VFR ( 1986cd4 )

Notes

Previously we were depending on a calculated frame rate (r_frame_rate)
but this isn't set to a sensible value for VFR streams. Packet duration also
can't be relied on (see the new VFR test case where each duration is N/A),
so we need to do some manual bookkeeping to calculate the timestamp
difference between frames ourselves.

I haven't yet fully understood what happens with the FPS filter during the
transition between segments but the behavior seems okay and the
timestamps don't change too much, aside from the +/- 1 frame difference.

Most unit tests pass on my machine, except I did not test GPU or non-H264
codecs. I don't think things would be much different there, but someone with
a GPU should check. The tests in ffmpeg/sign_test.go all timed out for me
and there is a segmenter test that seems flaky, but these are probably unrelated.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 50.20216%. Comparing base (684658a) to head (1986cd4).
Report is 4 commits behind head on master.

Additional details and impacted files

@@                 Coverage Diff                 @@
##              master        #393         +/-   ##
===================================================
+ Coverage   49.88901%   50.20216%   +0.31315%     
===================================================
  Files             12          12                 
  Lines           1802        1484        -318     
===================================================
- Hits             899         745        -154     
+ Misses           852         689        -163     
+ Partials          51          50          -1     

see 12 files with indirect coverage changes

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see 12 files with indirect coverage changes

[j0sh]

Another quick note that I forgot to mention. I am not sure how this fix would be rolled-out operationally, but because this will produce different frame counts (and thus different timings), it is important that streams be "pinned" to the same version of LPMS if some transcoders are slow to update themselves. Otherwise there might be some breakage in between segment intervals, eg timestamp overlaps or gaps.

[leszko]

Another quick note that I forgot to mention. I am not sure how this fix would be rolled-out operationally, but because this will produce different frame counts (and thus different timings), it is important that streams be "pinned" to the same version of LPMS if some transcoders are slow to update themselves. Otherwise there might be some breakage in between segment intervals, eg timestamp overlaps or gaps.

Good point, thanks for mentioning it!