fix(webgl): normalize nearly identical vertices before tessellation

[avinxshKD]

Resolves #8186

Changes

This PR fixes tessellation artifacts that occur when drawing shapes with multiple contours where consecutive vertices have nearly identical coordinates (differing by ~1e-8 or similar small amounts). This commonly happens when using font.textToContours().

Implementation Details

• Modified _tesselateShape() in src/webgl/p5.RendererGL.Immediate.js:

  • Added coordinate normalization loop before passing vertices to libtess tessellator
  • Consecutive vertices with coordinates within 1e-6 (epsilon) of each other are normalized to use the exact same value
  • Prevents numerical precision issues in libtess that cause glitchy rendering

• Added comprehensive unit test in test/unit/webgl/p5.RendererGL.js:

  • Tests tessellation with nearly identical consecutive vertices
  • Verifies that contour holes are properly cut out
  • Confirms shape fills work correctly with problematic coordinates

Technical Background

This workaround addresses a numerical precision issue in libtess, which is a JavaScript port of SGI C code from the 1990s. When consecutive vertices have coordinates that are almost (but not exactly) equal, libtess produces glitchy tessellation output. This issue is particularly likely to occur with contours automatically sampled from fonts via font.textToContours().

Screenshots of the change

Before:

• Tessellation produces visual artifacts and incorrect geometry
• Referenced in issue [p5.js 2.0 Bug Report]: Tessellation of beginShape/endShape with multiple contours sometimes produces weird results #8186

After (with this fix):

• Clean tessellation with proper contour cutouts
• No visual artifacts

PR Checklist

• npm run lint passes
• Inline reference is included / updated (N/A - internal fix, no API changes)
• Unit tests are included / updated

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Thank You!

[avinxshKD]

hey @davepagurek, kindly review when you get a chance