WIP: mid segment switch

A work-in-progress, don't merge.
I'm not sure this is correct either.
the `currentSegmentStartTime` and `nextSegmentDuration` variables are in seconds, while `playerCurrentTimeInMilliseconds` is in ms and that is what `getSwitchBuffer` is supposed to return.
Also, why does this include the duration of the next segment? We haven't downloaded it yet. I think it should just be `currentSegmentRemainingPlaybackTime`.