Re-use computeLuminance in the DOMFilterFactory.prototype.addHCMFilter method

[Snuffleupagus]

When the addHCMFilter method was added the helper function didn't exist, but now we can utilize it to avoid a tiny bit of code duplication.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.19%. Comparing base (0f26334) to head (2c57dec).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21813      +/-   ##
==========================================
- Coverage   90.20%   90.19%   -0.01%     
==========================================
  Files         264      264              
  Lines       67339    67337       -2     
==========================================
- Hits        60741    60734       -7     
- Misses       6598     6603       +5     

Flag	Coverage Δ
browsertest	66.29% <100.00%> (-0.05%)	⬇️
fonttest	8.91% <ø> (ø)
integrationtest	69.38% <100.00%> (-0.02%)	⬇️
unittest	58.36% <0.00%> (+<0.01%)	⬆️
unittestcli	56.93% <0.00%> (+<0.01%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

☔ View full report in Codecov by Harness.
📢 Have feedback on the report? Share it here.

🚀 New features to boost your workflow:

• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.
• 📦 JS Bundle Analysis: Save yourself from yourself by tracking and limiting bundle sizes in JS merges.