feat(captions): write in-band captions from DASH fmp4 segments to the textTrack API

[ldayananda]

Description

Depends on videojs/mux.js#197.

This will parse captions from DASH fmp4 segments as each segment request is finished and add them to a corresponding text track(i.e CC1, CC2, CC3, CC4).

Specific Changes proposed

• requires an update to the mux.js dependency

Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Unit Tests updated or fixed
• Reviewed by Two Core Contributors

[ldayananda]

This has been updated to use a persistent CaptionsParser. It still needs:

• to be updated to parse the Init segment only when necessary
• to reset the CaptionsParser and underlying CaptionStream on rendition switches e.c.t
• unit tests
  - [ ] docs if necessary

[ldayananda]

This should be ready for review, the tests fail because the Mux.js PR hasn't been merged and released yet.

I don't think there is anything needed for documentation here?