Added more comprehensive releaseGLObjects(State*=0) throughout Nodes,

Drawables,StateSet, and osgDB::Registry.

Added cleanup_frame() from to osgProducer::OsgCamerGroup to help with proper
clean of OpenGL objects before exit, and modified osgviewer, osghangglider,
osgwindows examples to do the extra frame call to cleanup_frame() before exit.

include/osg/Geode

@@ -132,6 +132,11 @@ class OSG_EXPORT Geode : public Node
             return _bbox;
         }
 
+        /** If State is non-zero, this function releases any associated OpenGL objects for
+           * the specified graphics context. Otherwise, releases OpenGL objexts
+           * for all graphics contexts. */
+        virtual void releaseGLObjects(osg::State* = 0) const;
+
     protected:
 
         virtual ~Geode();