Support timeShiftBufferDepth value of zero

[tdrews]

The DASH spec. indicates that a particular segment, S, becomes available at

S_start = availabilityStartTime + (start time of Period) +
          (MPD start time of S) + (duration of S)

and ceases to be available at

S_end = S_start + (duration of S) + timeShiftBufferDepth

(note: these calculations may be affected by other parameters, e.g., @suggestedPresentationDelay).
This indicates that every segment is available for at least its duration.
In Shaka v1.0 we have issues when timeShiftBufferDepth is 0 (#296),
we should handle small timeShiftBufferDepth values in v2.0.

[joeyparrish]

Due to the complexity of implementation, we will not support timeShiftBufferDepth of zero.