Volume Rendering: don't crash if a Chunk object gets destroyed after Root has been shutdown
Volume Rendering: don't crash if a Chunk object gets destroyed after Root has been shutdown

--HG--
branch : v1-9

jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre,jhu-lcsr-forks/ogre