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/osgProducer/OsgSceneHandler.cpp

@@ -25,6 +25,7 @@ OsgSceneHandler::OsgSceneHandler( osg::DisplaySettings *ds) :
 {
     _frameStartTick = 0;
     _previousFrameStartTick = 0;
+    _flushOfAllDeletedGLObjectsOnNextFrame = false;
 }
 
 void OsgSceneHandler::init()