ffmpeg: Drop preroll audio prior to muxing.
ffmpeg: Drop preroll audio prior to muxing.

Preroll audio is typically represented as a negative ts assuming
a starting offset of approximately zero, but our segments start
where earlier segments left off, compounding the timestamps.

The preroll behavior causes the ts to go backwards from the starting
timestamp. Since we don't have a way to signal preroll frames in
mpegts, the ffmpeg muxer would complain each time there was a
timestamp overlap.

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