From Michael Platings and Robert Osfield, added support for controlling,

via StateSet::setNestedRenderBin(bool) whether the new RenderBin should be nested
with the existing RenderBin, or be nested with the enclosing RenderStage.

src/osgUtil/CullVisitor.cpp

@@ -140,6 +140,8 @@ void CullVisitor::reset()
     //
     
     CullStack::reset();
+    
+    _renderBinStack.clear();
 
     _numberOfEncloseOverrideRenderBinDetails = 0;