GSOC 26: render multi-material models per part

[Nixxx19]

part of my google summer of code 2026 project, "full texture support for .mtl files in p5.js". works toward #6924.

this is phase 3: the renderer side, building on the data + parser foundation in #8879. multi-material models now actually draw, one part per material, each with its own look. public api is unchanged, you still just call loadModel() and model().

project phases for context:

• phase 1: design + pipeline study
• phase 2: data model + obj/mtl parser (GSOC 26: multi-material model parts (obj/mtl) #8879)
• phase 3: per-part rendering (this PR)

what's in here:

• the renderer loops over a geometry's parts and draws each one
• each part applies its own material (fill, specular, ambient, shininess, texture) around its draw, restored after so nothing leaks between parts
• a single-material geometry is its own part, so that path renders exactly as before, byte for byte
• p5.GeometryPart gains what the renderer needs to treat it as a drawable (hasFillTransparency, userVertexProperties)

tests:

• a multi-material model (octa-color, 8 materials) draws one fill per part and passes the instance count through
• the existing obj+mtl diffuse-colour visual test still matches its reference screenshot
• full unit suite green, including the webgpu and visual (pixel-compare) tests

stacked on #8879, so it should go in after that one merges.

[p5-bot]

Continuous Release

CDN link

• https://raw.esm.sh/pr/p5@63355be/lib/p5.min.js

Published Packages

• https://pkg.pr.new/p5@63355be

Commit hash: c9c0cc4

Previous deployments

ef33f8e

• https://raw.esm.sh/pr/p5@40e6b39/lib/p5.min.js
• https://pkg.pr.new/p5@40e6b39

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