Improved robustness of slicing when the slicing plane

crosses a horizontal plane of an object exactly.
Should improve Github issues #831, #895, #1102

xs/src/libslic3r/Polyline.hpp

@@ -19,6 +19,8 @@ public:
     Polyline() {};
     Polyline(const Polyline &other) : MultiPoint(other.points) {}
     Polyline(Polyline &&other) : MultiPoint(std::move(other.points)) {}
+    explicit Polyline(const Points &points) : MultiPoint(points) {}
+    explicit Polyline(Points &&points) : MultiPoint(std::move(points)) {}
     Polyline& operator=(const Polyline &other) { points = other.points; return *this; }
     Polyline& operator=(Polyline &&other) { points = std::move(other.points); return *this; }
 	static Polyline new_scale(std::vector<Pointf> points) {