1) Implemented anchoring of infill lines to perimeters with length

limited anchors, while before a full perimeter segment was always
   taken if possible.
2) Adapted the line infills (grid, stars, triangles, cubic) to 1).
   This also solves a long standing issue of these infills producing
   anchors for each sweep direction independently, thus possibly
   overlapping and overextruding, which was quite detrimental
   in narrow areas.
3) Refactored cubic adaptive infill anchroing algorithm
   for performance and clarity.

src/libslic3r/Fill/FillRectilinear2.hpp

@@ -17,7 +17,16 @@ public:
     virtual Polylines fill_surface(const Surface *surface, const FillParams &params);
 
 protected:
+    // Fill by single directional lines, interconnect the lines along perimeters.
 	bool fill_surface_by_lines(const Surface *surface, const FillParams &params, float angleBase, float pattern_shift, Polylines &polylines_out);
+
+
+    // Fill by multiple sweeps of differing directions.
+    struct SweepParams {
+        float angle_base;
+        float pattern_shift;
+    };
+    bool fill_surface_by_multilines(const Surface *surface, FillParams params, const std::initializer_list<SweepParams> &sweep_params, Polylines &polylines_out);
 };
 
 class FillMonotonic : public FillRectilinear2