Kenneth Moreland authored Apr 19, 2018 and Li-Ta Lo committed Apr 25, 2018

Previously when an external project loaded VTK-m with find_package(VTKm)
and then tried to use anything with OpenGL, you would get compiler (and
probably linker) errors. The problem was that
VTKmRenderingContexts.cmake skipped over the loading of OpenGL libraries
because the vtkm_rendering_gl_context target was imported before this
code was ever called. Correct the problem by going through the motions
of checking OpenGL every time.