title: OL3 - ol.layer.Group - zIndex - why?
body: <p>Does the z-index of a layer group serve any functionality?</p>
  
  <p>As far as I can see, this property doesn't have any effect at all. When rendering a frame, the layer states is a flattened list, then sorted by z-index.</p>
  
  <p>For me this is perfectly fine behavior. Z-index ordering should be completely independent from the the grouping hierarchy. So I would rather see this property move from the ol.layer.Base class to the ol.layer.Layer class. Does this make sense?</p>
  
  <p>Just want to make sure, that when I build onto this functionality, behavior is as I expect, and is expected to remain this way in future versions :).</p>