fix: gracefully handle mixed-material OBJ models instead of crashing

[Nixxx19]

Summary

Fixes a crash in parseObj() (loading.js:655-658) that broke loading of real-world OBJ files exported from Blender, Sketchfab, and similar tools.

Root cause

The check used to detect inconsistent vertex coloring was:

// BEFORE (broken)
if (hasColoredVertices === hasColorlessVertices) {
  throw new Error('Model coloring is inconsistent...');
}

The === comparison fires in two completely different situations:

State	Meaning	Should crash?	Did crash?
both false	Model has no faces at all	❌	✅ Bug
both true	Some faces have material colors, some don't	❌	✅ Bug

Many standard OBJ exports have only some mesh groups with an explicit usemtl entry — this is valid OBJ syntax and common in practice.

Fix

Changed the condition to only fire when both flags are true (the genuinely mixed case), and replaced the thrown error with a console.warn + vertex color strip so the model still loads:

// AFTER (fixed)
if (hasColoredVertices && hasColorlessVertices) {
  console.warn('p5.js: This OBJ model has mixed material coloring...');
  model.vertexColors = [];
}

Accessibility impact

Beginners loading common 3D assets from Sketchfab or Blender would get an opaque crash message with no path forward. This fix makes those models load and render with the default fill color, keeping the loadModel() experience frustration-free.

Test changes

The existing test 'inconsistent vertex coloring throws error' is updated to assert that a mixed-material model loads successfully and returns a Geometry with vertexColors.length === 0.

All 13 loadModel unit tests pass ✅

Test plan

• npx vitest run test/unit/io/loadModel.js — all 13 tests pass
• Mixed-material model (eg1.obj) now loads successfully and emits a console.warn
• Fully-colored models and no-material models are unaffected
• No changes to public API

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