combine fbo and fbo2 into one class and clean it up
having two VBO classes was confusing. 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 FBOs is updated in this topic to use the merged FBO class. Mainly this impacts classes that were using the FBO class. The old FBO methods and code are converted but commented out in this commit.
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