_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 / THREE.Object3D resources. These resources
can be reused, or "instantiated", at multiple nodes in the scene
hierarchy. THREE.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.