OPT: better way to inverse back wm seg to gm seg
OPT: better way to inverse back wm seg to gm seg

Former-commit-id: 7271e3c6f7863d325706b8c90de3ab32f2933854
Former-commit-id: 4139f67bdd103df99f21955e274b780dd7adabbf
3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox,3324fr/spinalcordtoolbox