function updateRegionHeights(region, minimumHeight, maximumHeight) {
  if (defined(minimumHeight)) {
    region[4] = minimumHeight;
  }

  if (defined(maximumHeight)) {
    region[5] = maximumHeight;
  }
}
For a bounding region, update the minimum and maximum height. This
is typically used to tighten a bounding volume using the
<code>TILE_MINIMUM_HEIGHT</code> and <code>TILE_MAXIMUM_HEIGHT</code>
semantics. Heights are only updated if the respective
minimumHeight/maximumHeight parameter is defined.

@param {Array} region A 6-element array describing the bounding region
@param {number} [minimumHeight] The new minimum height
@param {number} [maximumHeight] The new maximum height
@private