feat: support vtt regions

[gkatsev]

This depends on PR https://github.com/videojs/vtt.js/pull/50 and would need a new vttjs release. The change is fairly simple and stays within the current API constraints of vttjs. The only change necessary for regions is to not clear the display ourselves but let vttjs handle it now.

TODO

• [ ] https://github.com/videojs/vtt.js/pull/50
• [ ] backwards compatible to older versions of vtt.js

[stale[bot]]

This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.