MediaStoreRecorderCut.startBehindLiveMs property

Instead of startDateTime: start this many milliseconds before where the selected streams have recorded to (the earliest of their ends, in the newest session that recorded any of them), found as the player or cut opens. It is taken from the media’s own times, so neither the client’s clock nor the engine’s comes into it. A player started closer to live than the store can give media (a GOP or an audio fragment, whichever is longer) waits for it.

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