improve read_polygon_mesh_for_cdt_3

I have added comments on the block of code that triangulates the polygon on-the-fly

Constrained_triangulation_3/include/CGAL/Constrained_triangulation_3/internal/cdt_debug_io.h

@@ -135,19 +135,27 @@ namespace CGAL {
     // check if the polygon soup is pure triangles, and create a triangulated copy otherwise
     bool is_pure_triangles = std::all_of(faces.begin(), faces.end(), [](const Face &f) { return f.size() == 3; });
 
-     // create a non-deleting pointer to `faces` (with a null deleter)
-    using Deleter_function = void(Faces*);
-    using Deleter = Deleter_function*;
-    using Ptr = std::unique_ptr<Faces, Deleter>;
-    Ptr triangle_faces_ptr{&faces, +[](Faces *) {}};
-    if (!is_pure_triangles)
-    {
-      triangle_faces_ptr = Ptr(new Faces(faces), +[](Faces* vector){ delete vector; }); // copy `faces`
-      PMP::triangulate_polygons(points, *triangle_faces_ptr, np);
+    { // Now, call does_triangle_soup_self_intersect.
+      // ... but that function requires a triangulated soup (triangle soup)
+      // So, if needed, create a copy of the range of faces, and triangulate it on the fly.
+
+      // create a non-deleting pointer to `faces` (with a null deleter)
+      using Deleter_function = void(Faces*);
+      using Deleter = Deleter_function*; // function pointer type
+      using Ptr = std::unique_ptr<Faces, Deleter>;
+      auto null_deleter = +[](Faces *) {};
+      Ptr triangle_faces_ptr{&faces, null_deleter};
+      if (!is_pure_triangles)
+      {
+        auto faces_copy_ptr = new Faces(faces); // copy `faces`
+        auto delete_function_ptr = +[](Faces* vector){ delete vector; };
+        triangle_faces_ptr = Ptr(faces_copy_ptr, delete_function_ptr);
+        PMP::triangulate_polygons(points, *triangle_faces_ptr, np);
+      }
+
+      result.polygon_soup_self_intersects = PMP::does_triangle_soup_self_intersect(points, *triangle_faces_ptr, np);
     }
 
-    result.polygon_soup_self_intersects = PMP::does_triangle_soup_self_intersect(points, *triangle_faces_ptr, np);
-
     if (!PMP::orient_polygon_soup(points, faces))
     {
       result.polygon_mesh_is_manifold = false;