Account for the Firefox font size quantization in the text layer

[calixteman]

Firefox rounds the font size set on a canvas 2d context, once divided by the device pixel ratio, to 7 bits of precision. Hence measureText can return a width up to ~1% off the one the spans are laid out with, which made --scale-x slightly wrong in a zoom dependent way. Measure with a size left untouched by that rounding and rescale the returned width, which is linear in the font size.

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.19%. Comparing base (0f26334) to head (cb79d37).

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21810      +/-   ##
==========================================
- Coverage   90.20%   90.19%   -0.01%     
==========================================
  Files         264      264              
  Lines       67339    67345       +6     
==========================================
+ Hits        60741    60743       +2     
- Misses       6598     6602       +4     

Flag	Coverage Δ
browsertest	66.30% <90.90%> (-0.03%)	⬇️
fonttest	8.91% <ø> (ø)
integrationtest	69.40% <100.00%> (+<0.01%)	⬆️
unittest	58.36% <90.90%> (-0.01%)	⬇️
unittestcli	56.93% <0.00%> (+<0.01%)	⬆️

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