VR: Fix clipping of volumes in Clip mode
Volumes are not clipped in Clip interaction mode (VTKIS_CLIP).
Caused by 351c38b2 that only add clipping planes to vtkActor (vtkVolume is not a vtkActor subclass).
Remove casts to vtkActor in ForeachNonWidgetProp and StartClip/EndClip to fix.