Previously vtkm_find_gl was only usable when rendering was enabled.
This is problematic as examples would use vtkm_find_gl even if
they didn't use vtkm rendering library.
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.
