This approach unifies much of the code handling the clean up of OpenGL graphics data, avoids lots of local mutexes and static variables that were previously required,
and enables the clean up scheme to be easily extended by users providing their own GraphicsObjectManager subclasses.
git-svn-id: http://svn.openscenegraph.org/osg/OpenSceneGraph/trunk@15130 16af8721-9629-0410-8352-f15c8da7e697
