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.