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

var/spack/repos/builtin/packages/mesa/package.py

@@ -46,6 +46,10 @@ class Mesa(AutotoolsPackage):
     version('12.0.6', '1a3d4fea0656c208db59289e4ed33b3f')
     version('12.0.3', '1113699c714042d8c4df4766be8c57d8')
 
+    provides('gl@:4.5', when='@17:')
+    provides('gl@:4.4', when='@13:')
+    provides('gl@:4.3', when='@12:')
+
     variant('swrender', default=True,
             description="Build with (gallium) software rendering.")
     variant('hwrender', default=False,