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/FillBase.cpp

@@ -25,6 +25,7 @@ Fill* Fill::new_from_type(const InfillPattern type)
     case ipLine:                return new FillLine();
     case ipGrid:                return new FillGrid2();
     case ipTriangles:           return new FillTriangles();
+    case ipStars:               return new FillStars();
     case ipCubic:               return new FillCubic();
 //  case ipGrid:                return new FillGrid();
     case ipArchimedeanChords:   return new FillArchimedeanChords();