getIndex( renderObject ) {

		const { geometry, material } = renderObject;

		let index = geometry.index;

		if ( material.wireframe === true ) {

			const wireframes = this.wireframes;

			let wireframeAttribute = wireframes.get( geometry );

			if ( wireframeAttribute === undefined ) {

				wireframeAttribute = getWireframeIndex( geometry );

				wireframes.set( geometry, wireframeAttribute );

			} else if ( wireframeAttribute.version !== getWireframeVersion( geometry ) ) {

				this.attributes.delete( wireframeAttribute );

				wireframeAttribute = getWireframeIndex( geometry );

				wireframes.set( geometry, wireframeAttribute );

			}

			index = wireframeAttribute;

		}

		return index;

	}
Returns the index of the given render object's geometry. This is implemented
in a method to return a wireframe index if necessary.

@param {RenderObject} renderObject - The render object.
@return {?BufferAttribute} The index. Returns `null` for non-indexed geometries.