updateUsedTiles(usedTiles, tileSource, tile) {
    // FIXME should we use tilesToDrawByZ instead?
    const tileSourceKey = getUid(tileSource);
    if (!(tileSourceKey in usedTiles)) {
      usedTiles[tileSourceKey] = {};
    }
    usedTiles[tileSourceKey][tile.getKey()] = true;
  }
@param {!Object<string, !Object<string, boolean>>} usedTiles Used tiles.
@param {import("../../source/Tile.js").default} tileSource Tile source.
@param {import('../../Tile.js').default} tile Tile.
@protected