Fix #651 mrview: switching images resets slice increment in light box
Fix #651 mrview: switching images resets slice increment in light box

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