Also added some comments to gl-es drivers about missing polygon offset code.
The merges had some changes for that in the OpenGL driver, but both es drivers have no code at all about this.
git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/branches/ogl-es@5889 dfc29bdd-3216-0410-991c-e03cc46cb475