_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.