Refactoring. Use Model class for representing the plate in GUI

lib/Slic3r/Print/Object.pm

@@ -118,8 +118,7 @@ sub slice {
     # process facets
     for my $region_id (0 .. $#{$self->meshes}) {
         my $mesh = $self->meshes->[$region_id] // next;  # ignore undef meshes
-        $mesh->repair;
-        
+
         {
             my $loops = $mesh->slice([ map $_->slice_z, @{$self->layers} ]);
             for my $layer_id (0..$#$loops) {