Derive COctTreeSceneNode from IMeshSceneNode instead ISceneNode, and have it return the IMesh that was used to create its octtree.  Tested with a slightly modified example 07.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2173 dfc29bdd-3216-0410-991c-e03cc46cb475