multi-track support upstream to the ChunkSource interface.
This change does not yet make use of the newly exposed APIs. This
will come in a subsequent CL.
Issue #514.
- Currently all subtitles we parse contain timestamps relative to the sample
timestamp, however we add the sample timestamp in inconsistent ways (sometimes
in the Subtitle, sometimes in the SubtitleParser). This change converges on
a single approach. It also paves the way for passing absolute offsets to use
instead, and being able to apply them in a consistent way in a single place
(PlayableSubtitle). This functionality will be required for ISO 14496-30 TTML
embedded subtitles.
Issue: #689
