Add parallel cues wvtt option to timed subtitles

[tobbee]

No description provided.

[tobbee]

For lines with different alignments, one must have two parallel vttc cues in wvtt samples.
This is a case which is normally rarely tested and may not work well in all players (including dash.js).

Implement this a region option for timed subtitles. (For consistency, it may be good to implement it for stpp as well)

[tobbee]

To add this to the subtitle generation code turned out harder than I thought and is not specific to live streaming, so it is down-prioritised.