Configurable with --transport_stream_offset_ms.
This is needed to compensate for possible negative timestamps in
inputs, which could happen on ISO-BMFF with EditLists.
Issue #112.
Change-Id: I0fce8766c9df2911b9bb859c1e54052a8ed2abfb
Before, the webvtt output handler was written so that it could
share code between a segmented and non-segmented handler. As
we are not worried about that right now, this change simplifies
the handler to just be about segmented output.
Change-Id: I29dbc4e3a4ffbeb7ea10e23db489ee74b398a6c4