New label for Virtual Reality build

[banesullivan]

Would it be possible to add a new label (perhaps vtk_vr) much like the new vtk_dev label to build the VTK python bindings with the VTK_MODULE_ENABLE_VTK_RenderingOpenVR flag enabled to build the OpenVR modules?

I believe (though I am not sure) that this is only supported on Windows.

Reference:

• https://discourse.vtk.org/t/python-rendering-scene-to-vr/340
• https://github.com/pyvista/pyvista/issues/196

ping @MBrandt-NASA