Enhanced documentation for isFrontFacing to make it safer to use. Fixed frustum culling to correctly handle all possible cases of box intersections. Replaced isFrontFacing by classifyPointRelation as suggested by vitek. Minor cleanups in some classes.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@667 dfc29bdd-3216-0410-991c-e03cc46cb475