If the `Subtitle` has 'active' cues at `OutputOptions.startTimeUs`, this
change ensures these are emitted in a `CuesWithTiming` with
`CuesWithTiming.startTimeUs = OutputOptions.startTimeUs`. If
`OutputOptions.outputAllCues` is also set, then another `CuesWithTiming`
is emitted at the end that covers the 'first part' of the active cues,
and ends at `OutputOptions.startTimeUs`.
As well as adding some more tests to `LegacySubtitleUtilWebvttTest`,
this change also adds more tests for `TtmlParser` handling of
`OutputOptions`, which transitively tests the behaviour of
`LegacySubtitleUtil`.
#minor-release
PiperOrigin-RevId: 573151016
f9ece88a25