tckmap: Tweak DEC image output
tckmap: Tweak DEC image output

Use better intensity scaling for DEC images.

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