Fix of "Don't use bridging perimeters on top of elephant-foot compensation #3011"

If the Elephant foot compensation is applied to the 1st object's layer,
the uncompensated 1st object's slice is newly used for calculation
of bridges, overhans, skirt, brim, raft and supports.

Layer::slices were renamed to Layer::lslices to simplify reading
of the code, to differentiate from LayerRegion::slices.

src/libslic3r/Print.hpp

@@ -182,7 +182,7 @@ private:
 
     void _slice(const std::vector<coordf_t> &layer_height_profile);
     std::string _fix_slicing_errors();
-    void _simplify_slices(double distance);
+    void simplify_slices(double distance);
     bool has_support_material() const;
     void detect_surfaces_type();
     void process_external_surfaces();