mrview: lightbox: Always update slice_focus_delta
mrview: lightbox: Always update slice_focus_delta

* Even when cycling through vols this needs to be updated
MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3