This changes the default MP4 output to use TTML and adds a way to
choose which one is used. This is done with 'format=ttml+mp4' or
'format=vtt+mp4'.
This also fixes the boxes output in WebVTT in MP4.
Change-Id: Ieaa7fc44fbf4dc020a5bb70cfa3578ec10e088ce
Note that TTML in ISO-BMFF is not supported yet.
Also updated packager_test.py:
- Added a test using TTML passthrough.
- Computed output extension from input extension unless output_format
is specified.
Fixes#478.
Change-Id: Ia917fc4ed3c326782791ed67601fba02ea28b11d