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.

examples/osgwindows/osgwindows.cpp

@@ -159,6 +159,12 @@ int main( int argc, char **argv )
     // wait for all cull and draw threads to complete before exit.
     viewer.sync();
 
+    // run a clean up frame to delete all OpenGL objects.
+    viewer.cleanup_frame();
+
+    // wait for all the clean up frame to complete.
+    viewer.sync();
+
     return 0;
 }