UI and scenenodes are often connected. And while it was possible to work around this already by using custom draw functions
or deriving from gui and scene-nodes at the same time, it did already lead a few times to uglier code for me.
So I guess adding one more pass to the engine has it's uses.
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@6107 dfc29bdd-3216-0410-991c-e03cc46cb475