vtkPlaneWidget: fix zero movement issues
vtkPlaneWidget: fix zero movement issues

When a point is moved by a zero amount a divide
by zero can occur which shoudl not.

keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK,SimVascular/VTK,keithroe/vtkoptix,keithroe/vtkoptix,SimVascular/VTK,keithroe/vtkoptix,SimVascular/VTK