Set default VTK backend to OpenGL2
Created by: msmolens
Now that upstream VTK includes fixes for OpenGL2 GPU volume renderer issues that affected Slicer, update VTK and set the default VTK backend to OpenGL2.
Note: Following vtk/vtk!1790 (merged) (ENH: Introduce vtkMTimeType), some extensions will need to be updated using the procedure described in that merge request. Affected extensions include, at least:
- ModelToModelDistance
- SlicerIGT
- SlicerRT