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/osgViewer/api/Win32/GraphicsWindowWin32

@@ -58,7 +58,7 @@ class OSGVIEWER_EXPORT GraphicsWindowWin32 : public osgViewer::GraphicsWindow, p
         virtual void swapBuffersImplementation();
         
         /** Check to see if any events have been generated.*/
-        virtual void checkEvents();
+        virtual bool checkEvents();
 
         /** Set the window's position and size.*/
         virtual bool setWindowRectangleImplementation(int x, int y, int width, int height);