fix: fmp4 captions regression

[adrums86]

Description

Fixes an issue where caption data wasn't being passed out to the transmuxer callback for parsing.

Specific Changes proposed

Use the correctly scoped data object.

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

Merging #1434 (e127b7c) into main (dd5e2af) will not change coverage. The diff coverage is 100.00%.

@@           Coverage Diff           @@
##             main    #1434   +/-   ##
=======================================
  Coverage   85.98%   85.98%           
=======================================
  Files          42       42           
  Lines       10428    10428           
  Branches     2412     2412           
=======================================
  Hits         8967     8967           
  Misses       1461     1461           

Files	Coverage Δ
src/media-segment-request.js	96.11% <100.00%> (ø)

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