getTileSize(z) {
    if (this.tileSizes_) {
      return this.tileSizes_[z];
    }
    if (this.tileSize_) {
      return this.tileSize_;
    }
    const tileGrid = this.getTileGrid();
    return tileGrid ? toSize(tileGrid.getTileSize(z)) : [256, 256];
  }
Get the source tile size at the given zoom level.  This may be different than the rendered tile
size.
@protected
@param {number} z Tile zoom level.
@return {import('../size.js').Size} The source tile size.