getLoadedTilesFor(tiledImage) {
            if (!tiledImage) {
                return [...this._tilesLoaded];
            }
            return this._tilesLoaded.filter(tile => tile.tiledImage === tiledImage);
        }
Returns reference to all tiles loaded by a particular
tiled image item
@param {OpenSeadragon.TiledImage|null} tiledImage if null, gets all tiles, else filters out tiles
  that belong to a specific image