Added another texture type for OpenGL, namely the FBODepthRenderbuffer. This is only used internally for now, and also is still WIP as the current design has some bad smell :-(. However, it should allow for depthbuffer reuse for RTTs once the interface is made public.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1672 dfc29bdd-3216-0410-991c-e03cc46cb475