addFontToPile(pile, options) {
    if (options === undefined) return;
    if (options.font === undefined || options.font === null) return;

    const item = options.font;
    pile.push(item);
  }
Add given font option object to the list of objects (the 'pile') to consider for determining
multi-font option values.
@param {Pile} pile  pile of option objects to use
@param {object} options  instance to add to pile
@private