Cleanup of some method signatures and of XS return types

xs/xsp/ExPolygonCollection.xsp

@@ -26,6 +26,7 @@
     void simplify(double tolerance);
     Polygons polygons()
         %code{% RETVAL = *THIS; %};
+    Clone<Polygon> convex_hull();
 %{
 
 ExPolygonCollection*
@@ -76,13 +77,5 @@ ExPolygonCollection::append(...)
             THIS->expolygons.push_back(expolygon);
         }
 
-Polygon*
-ExPolygonCollection::convex_hull()
-    CODE:
-        RETVAL = new Polygon ();
-        THIS->convex_hull(RETVAL);
-    OUTPUT:
-        RETVAL
-
 %}
 };