function clone(extent, dest) {
  if (dest) {
    dest[0] = extent[0];
    dest[1] = extent[1];
    dest[2] = extent[2];
    dest[3] = extent[3];
    return dest;
  }
  return extent.slice();
}
Creates a clone of an extent.

@param {Extent} extent Extent to clone.
@param {Extent} [dest] Extent.
@return {Extent} The clone.