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.

src/osg/Drawable.cpp

@@ -490,6 +490,8 @@ void Drawable::compileGLObjects(State& state) const
 
 void Drawable::releaseGLObjects(State* state) const
 {
+    if (_stateset.valid()) _stateset->releaseGLObjects(state);
+
     if (!_useDisplayList) return;
     
     if (state)