mrdegibbs: switch from size_t to int
mrdegibbs: switch from size_t to int

Otherwise the command produces bad output, presumably due to wraparound
after subtraction of unsigned integers

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