Fall back to pixel-buffer SMask filtering when ctx.filter is unavailable

[calixteman]

For now OffscreenCanvas in worker threads doesn't support ctx.filter, so we need to fall back to a more expensive pixel-buffer SMask filtering in that case.
As a side effect, this also allows to support correctly smask in Safari.

[codecov-commenter]

Codecov Report

❌ Patch coverage is 0% with 20 lines in your changes missing coverage. Please review.
✅ Project coverage is 56.03%. Comparing base (a328294) to head (b823271).
⚠️ Report is 19 commits behind head on master.

Files with missing lines	Patch %	Lines
src/display/canvas.js	0.00%	20 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master   #21236      +/-   ##
==========================================
- Coverage   56.04%   56.03%   -0.01%     
==========================================
  Files         220      220              
  Lines       58973    58986      +13     
==========================================
+ Hits        33053    33055       +2     
- Misses      25920    25931      +11     

Flag	Coverage Δ
fonttest	8.65% <ø> (ø)
unittest	55.31% <0.00%> (-0.01%)	⬇️
unittestcli	54.56% <0.00%> (-0.02%)	⬇️

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[timvandermeij]

r=me; thanks!