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/Object

@@ -20,6 +20,9 @@
 
 namespace osg {
 
+// forward declare
+class State;
+
 /** META_Object macro define the standard clone, isSameKindAs and className methods.
   * Use when subclassing from Object to make it more convenient to define 
   * the standard pure virtual clone, isSameKindAs and className methods 
@@ -102,6 +105,11 @@ class OSG_EXPORT Object : public Referenced
         inline const Referenced* getUserData() const { return _userData.get(); }
 
 
+        /** 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:
 
         /** Object destructor. Note, is protected so that Objects cannot