function pickUrl(urls, z, x, y) {
  const hash = hashZXY(z, x, y);
  const index = modulo(hash, urls.length);
  return urls[index];
}
@param {Array<string>} urls List of URLs.
@param {number} z The tile z coordinate.
@param {number} x The tile x coordinate.
@param {number} y The tile y coordinate.
@return {string} The chosen URL.