_load(method, uri) {
      var r = this,
        p = r._loader[method](uri);
      if (!r._ready) {
        // re-render the scene when loading completes
        const call = r._call;
        r._ready = r._loader.ready().then(redraw => {
          if (redraw) call();
          r._ready = null;
        });
      }
      return p;
    }
Internal method for asynchronous resource loading.
Proxies method calls to the ImageLoader, and tracks loading
progress to invoke a re-render once complete.
@param {string} method - The method name to invoke on the ImageLoader.
@param {string} uri - The URI for the requested resource.
@return {Promise} - A Promise that resolves to the requested resource.