This adds VTK_REPLACE_OPENGL_OVERRIDES (off by default) as a build
option, if enabled the vtkRenderingOpenGL module will be modified to
depend on vtkRenderingOpenGL2 (privately) and its overrides modified
so that we can replace existing implementations piece by piece while
running the full suite of regression tests (and seeing how close the
drop in replacement classes are).

Change-Id: I6decb741bde86270c43578e1b24dca4058de6781