From Pjotr Svetachov, "when you restart threading

with startThreading/stopThreading the _drawQueue and _availableQueue
are not reset properly. This can lead to a deadlock when threading is
started again. So before threading is started again the queues must be
reset. This deadlock is also reported earlier by someone else in here:
http://forum.openscenegraph.org/viewtopic.php?p=43415#43415"

src/osgViewer/ViewerBase.cpp

@@ -336,6 +336,7 @@ void ViewerBase::startThreading()
         {
             renderer->setGraphicsThreadDoesCull(graphicsThreadsDoesCull);
             renderer->setDone(false);
+            renderer->reset();
             ++numViewerDoubleBufferedRenderingOperation;
         }
     }