Predict next segment in DASH SegmentTimeline

[joeyparrish]

It has come up many times that <SegmentTimeline> requires the manifest to be refreshed for the player to know about new segments. (See this issue comment or this list post.)

I don't recall who gave me this idea, and I can't find the conversation on github or in the mailing list, so it may have come up during a CWIP class. It should be feasible for us to predict what the next segment would look like in a way that is likely to be accurate enough to keep the user from buffering.

For example, if the last segment described in <SegmentTimeline> has a duration of 4096, we could simply assume the next one would be as well. Or we could be more conservative and assume that if the last N segments had the same duration, the next one would as well.

This would allow us to survive better with DASH content whose suggestedPresentationDelay and minimumUpdatePeriod parameters are not well-tuned.