New sparse infill: "stars" - David's star shaped infill.

This is very similar to a "triangles" infill, but maximum two lines
intersect at a single point.

added utility function get_extents_vector()

xs/src/libslic3r/Fill/FillRectilinear2.hpp

@@ -41,6 +41,17 @@ protected:
     virtual float _layer_angle(size_t idx) const { return 0.f; }
 };
 
+class FillStars : public FillRectilinear2
+{
+public:
+    virtual ~FillStars() {}
+    virtual Polylines fill_surface(const Surface *surface, const FillParams &params);
+
+protected:
+    // The grid fill will keep the angle constant between the layers, see the implementation of Slic3r::Fill.
+    virtual float _layer_angle(size_t idx) const { return 0.f; }
+};
+
 class FillCubic : public FillRectilinear2
 {
 public: