Update gl/qt options for vtk package (#6551)

This provides options for hardware and software rendering with the
gl API and updates the vtk package to make use of those options:

* Create new "gl" virtual package, provided by mesa
* Add external-only "opengl" package, which represents a system
  install that provides gl
* For vtk: prefer system gl implementation by default to get speed
  up from hardware rendering. When software rendering is specifically
  requested (+osmesa), try to use the llvmpipe approach, as it is
  much faster.
* Make qt dependency optional for vtk
* Add basic support for VTK on osx
* qt: Depend on virtual gl package, rather than mesa impl

etc/spack/defaults/packages.yaml

@@ -21,6 +21,7 @@ packages:
       blas: [openblas]
       daal: [intel-daal]
       elf: [elfutils]
+      gl: [mesa, opengl]
       golang: [gcc]
       ipp: [intel-ipp]
       java: [jdk]