Moved the exit on escape into Window, and added a virtual free method on both

Window an Viewer to clean up the windows, this is now called before exit is
finally called, ensure that more more memroy is clean up prior to exit.

include/osgGLUT/Viewer

@@ -96,6 +96,8 @@ class OSGGLUT_EXPORT Viewer : public Window, public osgUtil::GUIActionAdapter
 
     protected:
 
+        virtual void clear();
+
         virtual void display();
         virtual void reshape(GLint w, GLint h);
         virtual void mouseMotion(int x, int y);
@@ -142,6 +144,7 @@ class OSGGLUT_EXPORT Viewer : public Window, public osgUtil::GUIActionAdapter
         } times[3]; // store up to 3 frames worth of times
         bool _useDisplayLists;
         
+        
         osg::Timer   _timer;
         osg::Timer_t _tickRatePerSecond;
         osg::Timer_t _initialTick;