HLS uses the EXT-X-MEDIA-SEQUENCE tag at the start of a live playlist in
order to specify the first segment sequence number. This is because any
live playlist have a limited number of segments, and they also keep
updating with new segments while removing old ones. When a player refreshes
the playlist, this information is important for keeping track of segments
positions.
When the packager starts, it naturally starts this count from zero. However,
there are many situations where the packager may be restarted, without this
meaning starting this value from zero (but continuing a previous sequence).
The most common situations are problems in the encoder feeding the packager.
With those cases in mind, this parameter allows to set the initial
EXT-X-MEDIA-SEQUENCE value. This way, it's possible to continue the sequence
number from previous packager run.
Closes#691.
7aab7a8b50