adds an optional `--hls_start_time_offset` parameter, that, when
used, adds `EXT-X-START` tags to the HLS media playlists.
The EXT-X-START tag allows to specify the location where the player
starts playing, either from start (positive value) or from end (negative
value).
This is especially useful in case of livestreams where this tag can be
used to set the target latency of the playback.
Reference: https://datatracker.ietf.org/doc/html/rfc8216#section-4.3.5.2
The RFC says, that the `EXT-X-START` tag could also be added to the
master playlist, but my tests have shown that most players only respect
it when it's in the media playlists.
Fixes#970
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Co-authored-by: Joey Parrish <joeyparrish@google.com>
76eb2c1575