Cleanup fbo opengl2

having two FBO (really FO) classes was confusing not to mention places where folks were rolling their own FBO. This topic moves the functionality of FBO2 into FBO while keeping some of the original FBO functionality and cleaning it up a bit.

All OpenGL2 code in VTK using FBO/FBO2 is updated in this topic to use the merged FBO class. Mainly this impacts classes that were using the FBO class. FBO2 is currently left around as an empty subclass of FBO (probably should be removed, I can go either way on this)

Between being an internal class and the fact that the old FBO class had specific comments at the top of the file telling folks NOT to use it I think removing parts of the old API is worthwhile. The new code has examples of how to use it in the documentation. Maintaining both APIs in one class would make it confusing to new users which is the opposite of what this topic is trying to accomplish. I want using FBOs to be easy and consistent in VTK.