Legacy players, e.g. older versions of ExoPlayer, do not handle default webvtt text alignment correctly. Need to specify `align:center` explicitly cues without text alignment for backwards compatibility.
Fixes#925.
We currently have a bug about non-deterministic output in the MPD
generator. This works around that bug by optionally doing everything
in a single thread. This allows us to run manifest comparisons without
making the major changes needed to add that feature.
Issue #177
Change-Id: I10e1084dac77841220161fbd2575cdcb5c13c00e