As part of the CMake port we updated the duration formatting to contain
maximum of 6 decimal places but without trailing 0s. There was a bug
however where it used 6 significant digits rather than 6 decimal places
(`%g` rather than `%f`).
This fixes the bug and also updates the MPD sample files for the
integration tests to contain maximum of 6 decimal places.
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