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/qt/package.py

@@ -126,8 +126,7 @@ class Qt(Package):
     depends_on("python", when='@5.7.0:', type='build')
 
     # OpenGL hardware acceleration
-    depends_on("mesa", when='@4:+opengl')
-
+    depends_on("gl@3.2:", when='@4:+opengl')
     depends_on("libxcb", when=sys.platform != 'darwin')
     depends_on("libx11", when=sys.platform != 'darwin')