pointers for insert point on segment functions

Introduce pointers for functions that insert a point on a segment.
For L2, to keep the same functionality as before, the pointers
point to insert_exact_point_on_segment and insert_point_on_segment
functions.

Classes derived from Segment_Delaunay_graph_2 can change the
pointers to point to more specialized functions. For example, the
Linf implementation will point to different specialized function
defined in the Linf class.

Segment_Delaunay_graph_2/include/CGAL/Segment_Delaunay_graph_2.h

@@ -309,12 +309,35 @@ protected:
   CGAL_SEGMENT_DELAUNAY_GRAPH_2_NS::Internal::Which_list<Edge,List_tag>::List 
   List;
 
+
+protected:
+  // types for insert on segment functions
+  typedef Vertex_triple (Self::*Insert_on_Type)(
+      const Storage_site_2& ss, const Site_2& t,
+      Vertex_handle v, const Tag_true&);
+  Insert_on_Type insert_point_on_segment_ptr;
+
+  typedef Vertex_triple (Self::*Insert_Exact_on_Type)(
+      const Storage_site_2& ss, const Site_2& t,
+      Vertex_handle v);
+  Insert_Exact_on_Type insert_exact_point_on_segment_ptr;
+
+private:
+  // CREATION helper
+  void setup_insert_on_pointers_l2(void) {
+    insert_point_on_segment_ptr = &Self::insert_point_on_segment;
+    insert_exact_point_on_segment_ptr = &Self::insert_exact_point_on_segment;
+  }
+
 public:
   // CREATION
   //---------
   Segment_Delaunay_graph_2(const Geom_traits& gt = Geom_traits(),
 			   const Storage_traits& st = Storage_traits())
-    : DG(gt), st_(st) {}
+    : DG(gt), st_(st)
+  {
+    setup_insert_on_pointers_l2();
+  }
 
   template< class Input_iterator >
   Segment_Delaunay_graph_2(Input_iterator first, Input_iterator beyond,
@@ -322,6 +345,7 @@ public:
 			   const Storage_traits& st = Storage_traits())
     : DG(gt), st_(st)
   {
+    setup_insert_on_pointers_l2();
     insert(first, beyond);
   }
 
@@ -1061,11 +1085,11 @@ protected:
   Vertex_handle insert_point2(const Storage_site_2& ss,
 			      const Site_2& t, Vertex_handle vnear);
 
-  Triple<Vertex_handle,Vertex_handle,Vertex_handle>
+  Vertex_triple
   insert_point_on_segment(const Storage_site_2& ss, const Site_2& t,
 			  Vertex_handle v, const Tag_true&);
 
-  Triple<Vertex_handle,Vertex_handle,Vertex_handle>
+  Vertex_triple
   insert_exact_point_on_segment(const Storage_site_2& ss, const Site_2& t,
 				Vertex_handle v);
 

Segment_Delaunay_graph_2/include/CGAL/Segment_Delaunay_graph_2/Segment_Delaunay_graph_2_impl.h

@@ -338,7 +338,8 @@ insert_point(const Storage_site_2& ss, const Site_2& t,
     if ( at_res == AT2::INTERIOR ) {
       CGAL_assertion( t.is_input() );
 
-      Vertex_triple vt = insert_exact_point_on_segment(ss, t, vnearest);
+      Vertex_triple vt = (this->*insert_exact_point_on_segment_ptr)(
+          ss, t, vnearest);
       return vt.first;
     } else {
       // the point to be inserted does not belong to the interior of a
@@ -931,7 +932,7 @@ insert_intersecting_segment_with_tag(const Storage_site_2& ss,
     return v;
   }
 
-  Vertex_triple vt = insert_point_on_segment(ss, t, v, tag);
+  Vertex_triple vt = (this->*insert_point_on_segment_ptr)(ss, t, v, tag);
 
   Vertex_handle vsx = vt.first;
   
@@ -2778,6 +2779,11 @@ void
 Segment_Delaunay_graph_2<Gt,ST,D_S,LTag>::
 copy(Segment_Delaunay_graph_2& other)
 {
+  // copy the insert_on method pointers
+  insert_point_on_segment_ptr = other.insert_point_on_segment_ptr;
+  insert_exact_point_on_segment_ptr =
+    other.insert_exact_point_on_segment_ptr;
+
   // first copy the point container
   pc_ = other.pc_;