Fix packet synchronization bug #1806
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borovik135
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Nov 9, 2023
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This bug got missed in #1802 due to a typo in one of the unit tests causing it to run on the wrong video file. Run on the correct file, the unit test fails, alerting us that copying both the video and audio streams causes a desync between them. The bug may be fixed by adjusting how we adjust timestamps in accordance with
start_timestamp- instead of having FFmpeg shift all packets forward, then manually shifting back the ones that are direct from the original video (and therefore already takestart_timestampinto account), we tell FFmpeg to do nothing and manually shift forward packets that aren't from the original video (i.e. only video packets encoded by us). Alternately, we could have added backward shifting to the copied video packets and kept the rest as is, but this results in code duplication and is probably less clear anyway.