_addNodeRef(cache, index) {
    if (index === undefined) return;
    if (cache.refs[index] === undefined) {
      cache.refs[index] = cache.uses[index] = 0;
    }
    cache.refs[index]++;
  }
Counts references to shared node / Object3D resources. These resources
can be reused, or "instantiated", at multiple nodes in the scene
hierarchy. Mesh, Camera, and Light instances are instantiated and must
be marked. Non-scenegraph resources (like Materials, Geometries, and
Textures) can be reused directly and are not marked here.

Example: CesiumMilkTruck sample model reuses "Wheel" meshes.

@param {Object} cache
@param {Object3D} index