feat: parse mp4 webvtt segments

[adrums86]

Description

Feature

This pull request adds support for parsing mp4 encapsulated webvtt segments by modifying the existing vtt-segment-loader and associated functions to interface with the transmuxer to parse fmp4 webvtt initialization and content segments.

Specific Changes proposed

• Update mux.js to include the WebVttParser for use in the transmuxer worker.
• Modify the vtt-segment-loader to be able to handle fragmented mp4 segments.
• Parse wvtt initialization segments and content segments with the help of the transmuxer.
• Parse the cues provided by the transmuxer into VTTCue objects and add them to the subtitles text tracks.

Demo

See the deployed demo page here with the simulated live DASH manifest from DASH-IF with mp4 encapsulated webvtt: https://livesim2.dashif.org/livesim2/timesubswvtt_en/testpic_2s/Manifest.mpd

Requirements Checklist

• [x] Feature implemented / Bug fixed
• [ ] If necessary, more likely in a feature request than a bug fix
  • [ ] Unit Tests updated or fixed
  • [ ] Docs/guides updated
  • [ ] Example created (starter template on JSBin)
• [ ] Reviewed by Two Core Contributors

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 98.11321% with 1 line in your changes missing coverage. Please review.

Project coverage is 84.06%. Comparing base (8456cb3) to head (3618bdc). Report is 1 commits behind head on main.

Files with missing lines	Patch %	Lines
src/transmuxer-worker.js	80.00%	1 Missing :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #1545      +/-   ##
==========================================
- Coverage   86.39%   84.06%   -2.34%     
==========================================
  Files          43       43              
  Lines       11165    11627     +462     
  Branches     2552     2604      +52     
==========================================
+ Hits         9646     9774     +128     
- Misses       1519     1853     +334     

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