Fix: remove gradient color artifacts by earcut-based triangulation

[ZaildarAnmol]

Problem: gradient fills show extra colors due to overlapping triangle fan.
Fix: non-overlapping triangulation using mapbox_earcut (outer ring + holes).
Fallback to previous triangulation if dependency absent.
Tested on the minimal scene from issue #2387/#2382 and several concave/holed shapes; artifacts are gone.
No API changes; performance comparable for typical glyphs/shapes.

Fixes #2387

[Vegeorca]

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