Fix rendering of web Rect with large corner radius

[ianthomas23]

This fixes the rendering of WebGL Rect glyphs with large corner radii. Here is the Glyph_models__should_support_Rect_with_large_rounded_corners_that_need_scaling.png baseline test image before and after:

There isn't a simple Python reproducer until #13637 is fixed.

Code changes are all in the shaders. Previously we were only considering the distance to the nearest Rect corner so we only looked at the local x,y quadrant. This isn't sufficient and we have to look at all 4 corners to support large radii. This changes the logic of the fragment shader, necessarily making it more complicated. I have also moved the calculation to limit the corner radii to the vertex shader so that it only needs to be calculated 4 times (one per rect vertex) rather than once per pixel rendered, and written it in a more condensed and faster form.

• issues: fixes [BUG] WebGL Rect with large corner radius rendered incorrectly #13648
• tests added / passed
• release document entry (if new feature or API change)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (59751d9) 92.56% compared to head (2633d11) 92.56%.

Additional details and impacted files

@@             Coverage Diff             @@
##           branch-3.4   #13649   +/-   ##
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  Coverage       92.56%   92.56%           
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  Files             323      323           
  Lines           20509    20509           
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  Hits            18985    18985           
  Misses           1524     1524