feat(emulated-tracks): add class to force cues to be center aligned

[gkatsev]

videojs/http-streaming#1408 updated 608 captions to default to be left aligned. This may be unwanted by some folks and we should provide an easier way to force them to be centered. This PR adds a player level class that will override the text alignment to be center. It also overrides the width to 80% because otherwise the cue box isn't set up correctly to be 10% from the right of the display area (a side effect of hardcoding a width value and using inset in the generation of the cues).

Description

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Requirements Checklist

• Feature implemented / Bug fixed
• If necessary, more likely in a feature request than a bug fix
  • Change has been verified in an actual browser (Chrome, Firefox, IE)
  • Unit Tests updated or fixed
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  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.71%. Comparing base (d7757d8) to head (ca1ec65).

Additional details and impacted files

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  Coverage   82.71%   82.71%           
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  Files         113      113           
  Lines        7636     7636           
  Branches     1835     1835           
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  Hits         6316     6316           
  Misses       1320     1320           

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[wseymour15]

This looks like a fine change to me, assuming that this class setting will be optional. We made this change because according to the spec: "CEA-608 does not distinguish between different alignment, so all captions are left aligned and require a WebVTT align cue setting to override the default middle alignment".

We interpreted this to mean by default, 608 captions should be left aligned.

[gkatsev]

Yup, it's purely an opt-in class to make it easier to center. See some discussion in slack. I agree that the updated default is the correct one.