function extendFlatCoordinates(
  extent,
  flatCoordinates,
  offset,
  end,
  stride,
) {
  for (; offset < end; offset += stride) {
    extendXY(extent, flatCoordinates[offset], flatCoordinates[offset + 1]);
  }
  return extent;
}
@param {Extent} extent Extent.
@param {Array<number>} flatCoordinates Flat coordinates.
@param {number} offset Offset.
@param {number} end End.
@param {number} stride Stride.
@return {Extent} Extent.