StateSetUpdater: use the frameNumber

More robust in case a node is updated twice in the same frame (e.g. because it has multiple parents).

components/sceneutil/statesetupdater.cpp

@@ -20,11 +20,10 @@ namespace SceneUtil
             }
         }
 
-        // Swap to make the StateSet in [0] writable, [1] is now the StateSet that was queued by the last frame
-        std::swap(mStateSets[0], mStateSets[1]);
-        node->setStateSet(mStateSets[0]);
+        osg::StateSet* stateset = mStateSets[nv->getTraversalNumber()%2];
+        node->setStateSet(stateset);
 
-        apply(mStateSets[0], nv);
+        apply(stateset, nv);
 
         traverse(node, nv);
     }

components/sceneutil/statesetupdater.hpp

@@ -12,8 +12,7 @@ namespace SceneUtil
     ///     DYNAMIC data variance but that would undo all the benefits of the threading model - having the cull and draw
     ///     traversals run in parallel can yield up to 200% framerates.
     /// @par Race conditions are prevented using a "double buffering" scheme - we have two StateSets that take turns,
-    ///     the first StateSet is the one we can write to, the second is the one currently in use by the draw traversal of the last frame.
-    ///     After a frame is completed the places are swapped.
+    ///     one StateSet we can write to, the second one is currently in use by the draw traversal of the last frame.
     /// @par Must be set as UpdateCallback on a Node.
     /// @note Do not add the same StateSetUpdater to multiple nodes.
     /// @note Do not add multiple StateSetControllers on the same Node as they will conflict - instead use the CompositeStateSetUpdater.