Optional material override for Command3D.DrawModelWith.

All applies one Material3D to every sub-mesh (allocation-free struct field). PerMesh takes a resolver indexed by the pipeline's sub-mesh iteration order — mesh index 0..model.MeshCount-1 on the raylib backend.

The PerMesh resolver is invoked several times per mesh per frame (forward pass, shadow collection, and the per-frame shader-variant warm-up), with no memoization. It must be pure and cheap — prefer returning a precomputed material over allocating or computing per call.