getDrawIndex() {

		const extensions = this.renderer.backend.extensions;

		if ( extensions.has( 'WEBGL_multi_draw' ) ) {

			return 'uint( gl_DrawID )';

		}

		return null;

	}
Returns the draw index builtin.

@return {?string} The drawIndex shader string. Returns `null` if `WEBGL_multi_draw` isn't supported by the device.