title: how to HIDE a groundoverlay instead of removing it by calling setMap(null)
body: <p>I have multiple GroundOverlays loaded on the map and I try to keep them ordered. It seems the only thing I can do is to loaded (create the GroundOverlay object and setMap(map)) one by one (Please let me know if there are other better ways), whcih I can live with for now. However, I also have controls (checkboxes) to turn on/off a individual layer, which ends with the order is completely messed up. For instance, say I have L1, L2, L3 three layers ordered from bottom to top, if I turn off L2 and turn it on again, the L2 will be on the top of L3 as L2 was removed from the map when turned off.</p>
  
  <p>My first attempt is to find a way to manage the order of the GroundOverlays, which I failed (could not find any); The second attempt is to try if I can 'hide' the layer when turn it off instead of removing the layer when calling setMap(null).</p>
  
  <p>Any idear? Thanks!</p>