Bug fix draw Nef_3 (#7388)

_Please use the following template to help us managing pull requests._

## Summary of Changes

Draw only halffacet if(fc.is_shalfedge()).
TODO: do we need to draw something if (fc.is_shalfloop()) ?

## Release Management

* Affected package(s): Nef_3
* Issue(s) solved (if any): fix #7377

Nef_3/include/CGAL/draw_nef_3.h

@@ -130,6 +130,8 @@ protected:
       // Now iterate through holes of the face
       ++fc;
       while(fc!=f->facet_cycles_end())
+      {
+        if(fc.is_shalfedge())
         {
           se = SHalfedge_const_handle(fc);
           hc_start=se;
@@ -143,9 +145,9 @@ protected:
                                    viewer.get_vertex_normal(hc_start->source()->center_vertex()));
           viewer.add_point_in_face(lastvh->point(),
                                    viewer.get_vertex_normal(lastvh));
+        }
         ++fc;
       }
-
       viewer.face_end();
       facets_done[f]=true;
       n_faces++;