fix: Check for VTTCue

[wseymour15]

This was causing a bug when it came to VTT Cue positioning. This check was never true becasue the window instances of the VTTCue being checked, and the window instance here are never the same.

Specific Changes proposed

A different way to check if the property being used is indeed a VTTCue.

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
  • Docs/guides updated
  • Example created (starter template on JSBin)
• Reviewed by Two Core Contributors

[codecov]

Codecov Report

Merging #8370 (93d4139) into main (2b0df25) will decrease coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##             main    #8370      +/-   ##
==========================================
- Coverage   82.68%   82.65%   -0.04%     
==========================================
  Files         113      113              
  Lines        7561     7651      +90     
  Branches     1818     1867      +49     
==========================================
+ Hits         6252     6324      +72     
- Misses       1309     1327      +18     

Impacted Files	Coverage Δ
src/js/tracks/text-track.js	94.23% <100.00%> (-0.65%)	⬇️

... and 5 files with indirect coverage changes

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[wseymour15]

NOTE: Before merging, we need to ensure we pull in the new version of vtt.js including changes from this PR: videojs/vtt.js#65

Also, we do not need to update the version of VHS here, since technically VHS depends on Video.js in this instance.

[misteroneill]

LGTM, but converted to a draft in case it's still blocked. Feel free to mark it ready if that's erroneous.

[wseymour15]

we do not need to update the version of VHS here, since technically VHS depends on Video.js in this instance.

Thanks @misteroneill! In this case, we have all the dependencies we need, as VHS actually depends on the changes here. We will need to update the video.js version in VHS once this is deployed. I'll reopen this!