COMP: Add logic to selectively use OpenGL or OSMesa.

Use vtkOpenGL.cmake to decide to use (LINK_PRIVATE) OpenGL or Offscreen Mesa.
OpenGL is now LINK_PRIVATE, so additional libraries and tests need to include OpenGL.
Include the following in your CMakeLists.txt:

include(vtkOpenGL)
vtk_opengl_link(${module})

for every ${module} that uses OpenGL. This takes care of include directories and
linking of proper libraries.