MRView: ensure overlay image transparency is constant
MRView: ensure overlay image transparency is constant

Transparency setting was proportional to image intensity, which is
rarely the desired behaviour.

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