Handle regionId from template that already exists

[mfinger-irt]

When adding a new subtitle block the region for this block will be the same as the region defined in the template file (if one exists).
The question is: how to handle the case when there is already a region with the same id defined?

[andreastai]

This would actually make the IMSC instance not conformant, so this needs somehow to be fixed.

[andreastai]

I added a bug label as an error might occur without the option of the use to trace what the root of the problem was.

[andreastai]

It would indeed be more robust if we check for each generated region id if the value already exist in the TTML document. But because of how we generate the region id the probability that a duplicate region id is generated is quite low. Therefore I would give this issue a low priroity.