Added osg::CollectOccludersVisitor which is a helper class for finding active

occluder in the view frustum, to be used as pre cull traversal.

include/osg/Node

@@ -141,6 +141,12 @@ class SG_EXPORT Node : public Object
           * note, return true only if no children have culling disabled, and the local _cullingActive flag is true.*/
         inline const bool isCullingActive() const { return _numChildrenWithCullingDisabled==0 && _cullingActive && _bsphere.isValid(); }
 
+        /** Get the number of Children of this node which are or have OccluderNode's.*/
+        inline const int getNumChildrenWithOccluderNodes() const { return _numChildrenWithOccluderNodes; }
+
+        
+        /** return true if this node is an OccluderNode or the subgraph below this node are OccluderNodes.*/
+        inline const bool containsOccluderNodes() const;
 
         /**
          * Set user data, data must be subclased from Referenced to allow
@@ -248,6 +254,9 @@ class SG_EXPORT Node : public Object
         int _numChildrenWithCullingDisabled;        
         void setNumChildrenWithCullingDisabled(const int num);
 
+        int _numChildrenWithOccluderNodes;        
+        void setNumChildrenWithOccluderNodes(const int num);
+
         osg::ref_ptr<Referenced> _userData;
 
         NodeMask _nodeMask;