Optimization: simplify fill_surfaces before the offset operation

lib/Slic3r/Layer/Region.pm

@@ -209,9 +209,8 @@ sub make_perimeters {
         # and then we offset back and forth by the infill spacing to only consider the
         # non-collapsing regions
         push @{ $self->fill_surfaces },
-            map $_->simplify(&Slic3r::SCALED_RESOLUTION),
             offset2_ex(
-                    \@last,
+                [ map $_->simplify(&Slic3r::SCALED_RESOLUTION), @last ],
                 -($perimeter_spacing/2 + $infill_spacing),
                 +$infill_spacing,
             );