COMP: update VTK8 git tag
Including Clinck and Drag modification made in VTK: vtk/vtk!3160 (merged)
This is necessary for https://github.com/Slicer/Slicer/pull/770
Including Clinck and Drag modification made in VTK: vtk/vtk!3160 (merged)
This is necessary for https://github.com/Slicer/Slicer/pull/770