Per-face material selector. Called once per face group; return a material to
paint that face, or undefined to leave it at the glTF default. Consumed by
tooling (e.g. brepjs-cad's export const materials) that has the mesh and
builds the faceId→material map passed to GltfExportOptions.materials.
Per-face material selector. Called once per face group; return a material to paint that face, or
undefinedto leave it at the glTF default. Consumed by tooling (e.g. brepjs-cad'sexport const materials) that has the mesh and builds the faceId→material map passed to GltfExportOptions.materials.