Allow changing zbuffer enable and write in 2d override material.
Introduce NO_CLIP hints to d3d9 and opengl driver for 2d mode. Since we clip all 2d operations manually, it should be safe to tell the driver to disable clipping here. Might help on slower machines and is generally suggested by the hw vendors. We might introduce a mesh flag for this as well in the future, in order to disable clipping for larger geometry as well.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3736 dfc29bdd-3216-0410-991c-e03cc46cb475