Use Clipper for line clipping

xs/xsp/Clipper.xsp

@@ -91,6 +91,15 @@ diff_ex(subject, clip, safety_offset = false)
     OUTPUT:
         RETVAL
 
+Polylines
+diff_pl(subject, clip)
+    Polylines   subject
+    Polygons    clip
+    CODE:
+        diff(subject, clip, RETVAL);
+    OUTPUT:
+        RETVAL
+
 Polygons
 intersection(subject, clip, safety_offset = false)
     Polygons                    subject
@@ -111,6 +120,15 @@ intersection_ex(subject, clip, safety_offset = false)
     OUTPUT:
         RETVAL
 
+Polylines
+intersection_pl(subject, clip)
+    Polylines                   subject
+    Polygons                    clip
+    CODE:
+        intersection(subject, clip, RETVAL);
+    OUTPUT:
+        RETVAL
+
 ExPolygons
 xor_ex(subject, clip, safety_offset = false)
     Polygons                    subject

xs/xsp/Line.xsp

@@ -25,6 +25,8 @@
          %code{% const char* CLASS = "Slic3r::Point"; RETVAL = THIS->midpoint(); %};
     Point* point_at(double distance)
          %code{% const char* CLASS = "Slic3r::Point"; RETVAL = THIS->point_at(distance); %};
+    Polyline* as_polyline()
+        %code{% const char* CLASS = "Slic3r::Polyline"; RETVAL = new Polyline(); RETVAL->points.push_back(THIS->a); RETVAL->points.push_back(THIS->b); %};
 %{
 
 Line*

xs/xsp/PolylineCollection.xsp

@@ -17,6 +17,8 @@
         %code{% const char* CLASS = "Slic3r::Polyline::Collection"; RETVAL = THIS->chained_path_from(start_near, no_reverse); %};
     int count()
         %code{% RETVAL = THIS->polylines.size(); %};
+    Point* leftmost_point()
+        %code{% const char* CLASS = "Slic3r::Point"; RETVAL = THIS->leftmost_point(); %};
 %{
 
 PolylineCollection*

xs/xsp/my.map

@@ -22,6 +22,7 @@ ClipperLib::PolyFillType		T_UV
 Points          T_ARRAYREF
 Lines           T_ARRAYREF
 Polygons        T_ARRAYREF
+Polylines       T_ARRAYREF
 ExPolygons      T_ARRAYREF
 
 # we return these types whenever we want the items to be returned

xs/xsp/typemap.xspt

@@ -15,6 +15,7 @@
 %typemap{Points};
 %typemap{Lines};
 %typemap{Polygons};
+%typemap{Polylines};
 %typemap{ExPolygons};
 %typemap{Polygons*};
 %typemap{TriangleMeshPtrs};