Fixed invalidation of FDM back end if a custom per layer extruder change is added or removed.
Fixed invalidation of FDM back end if a custom per layer
extruder change is added or removed.

prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r,prusa3d/Slic3r