Add MRT support for OpenGL and Direct3D9. This patch is heavily based on the code from Nadro. This version supports also special render targets for OpenGL, and is already prepared for further MRT extensions such as per-target blend support. The d3d9 version is not yet tested.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2808 dfc29bdd-3216-0410-991c-e03cc46cb475