code: function handleFontLoad (fontFace) {
    _backingStores.forEach(function (backingStore) {
      if (layerContainsFontFace(backingStore.layer, fontFace)) {
        invalidateBackingStore(backingStore.id);
      }
    });
  }
docs: Invalidates the backing stores for layers which contain a text layer
  associated with the given font face.
  
  @param {FontFace} fontFace