Added support for an update OperationQueue to Viewer and CompositeViewer to allow asyncrnous adding of operations to be added

to the view to be done during syncronous updateTraversal().

This feature can be used for doing things like merging subgraphs that have been loaded
in a background thread.

include/osg/OperationThread

@@ -109,6 +109,9 @@ class OSG_EXPORT OperationQueue : public Referenced
 
         /** Remove all operations from OperationQueue.*/
         void removeAllOperations();
+        
+        /** Run the operations. */
+        void runOperations(Object* callingObject=0);
 
         /** Call release on all operations. */
         void releaseAllOperations();