New FDM support sparse infill zig-zag algorithm.

Fixed some old support and infill issues.

Fixes support problem #4295
Fixes Parts of interface layer extends beyond supports and cannot be printed
Fixes support missing under horizontal overhang #6058
Fixes Slicer double-traces small sections of Rectilinear Supports, causes
Fixes plastic buildup and nozzle crashes #4951
Fixes Add "Angle Interface layers" #2969

src/libslic3r/MultiPoint.hpp

@@ -64,6 +64,7 @@ public:
     bool has_duplicate_points() const;
     // Remove exact duplicates, return true if any duplicate has been removed.
     bool remove_duplicate_points();
+    void clear() { this->points.clear(); }
     void append(const Point &point) { this->points.push_back(point); }
     void append(const Points &src) { this->append(src.begin(), src.end()); }
     void append(const Points::const_iterator &begin, const Points::const_iterator &end) { this->points.insert(this->points.end(), begin, end); }