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/Node

@@ -248,6 +248,12 @@ class OSG_EXPORT Node : public Object
         void dirtyBound();
 
 
+        /** 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:
 
         /** Node destructor. Note, is protected so that Nodes cannot