Fixed panning bug, when using RUN_ON_DEMAND, that resulted in the camera being thrown off towards infinity.

The solution for to refactor the way that events are checked so I add a bool return type to checkEvents() method across osgViewer::GraphcisWindow, osgGA::Devive and osgViewer::Viewer/CompositeViewer classes

include/osgGA/EventQueue

@@ -35,6 +35,12 @@ class OSGGA_EXPORT EventQueue : public osg::Referenced
 
         typedef std::list< osg::ref_ptr<GUIEventAdapter> > Events;
 
+        bool empty() const
+        {
+            OpenThreads::ScopedLock<OpenThreads::Mutex> lock(_eventQueueMutex);
+            return _eventQueue.empty();
+        }
+        
         /** Set events.*/
         void setEvents(Events& events);