Add parallel cues wvtt option to timed subtitles

[tobbee]

[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)