When switching format in HLS, we instantiate a new extractor, which
adjusts TS presentation timestamps so that they align properly with
the start of the first segment in the new format. Some HLS streams
appear to have slightly misalignment that causes a glitch when using
this approach.
It's better to re-use the same timestamp adjustment across formats,
and only reset it when seeking or when there's an actual discontinuity.
This is because the HLS spec guarantees PTS timestamp alignment across
different formats.
We'll also need something like PtsTimestampAdjuster to share between
separated audio and WebVTT tracks, which also contain PTS timestamps
that are aligned, and will need to share a common adjustment.
Issue: #692
e48851d8cc