Added support for Stereo framebuffers and AUX buffers (which are currently only accessible via direct OpenGL calls or shaders).
Added a creation parameter for doublebuffering.
Cleaned up the framebuffer clean methods in OpenGL.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@2102 dfc29bdd-3216-0410-991c-e03cc46cb475