[vtkusers] vtkActor2D::LayerNumber and Overlays

[John Biddiscombe]

I'm having trouble getting Actor2D's to render over the top of each other 
in the correct order. I need one actor to be always on top of another, but 
below certain others etc etc. I can't always control the order they are 
adder to the renderer.

Is there a correct way of using Overlay planes - layernumber etc to achive 
the desired result, or do I need to subclass/modify vtkRenderer and insert 
a "SortActors" method somewhere after props are added?

Thanks

JB