Cleanup of some method signatures and of XS return types

xs/xsp/BoundingBox.xsp

@@ -16,8 +16,7 @@
     void scale(double factor);
     void translate(double x, double y);
     void offset(double delta);
-    Polygon* polygon()
-        %code{% RETVAL = new Polygon(); THIS->polygon(RETVAL); %};
+    Clone<Polygon> polygon();
     Clone<Point> size();
     Clone<Point> center();
     Clone<Point> min_point() %code{% RETVAL = THIS->min; %};