mrview: lightbox: make sure volume increment value persists even after toggling between view modes
mrview: lightbox: make sure volume increment value persists even after toggling between view modes

MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3,MRtrix3/mrtrix3