Fix #109236: Split Edges node skips loose edges

With the previous fix to the node, 8a11f0f3a2, the new edge
indices are build with the changed corner verts from a previous step in
the algorithm. That doesn't work for loose edges though, since they
aren't used by any face corners. The best solution I could come up with
was adding a second loop over the split vertices that adjusts the vertex
indices of loose edges. This can be skippe when there are none.

@@ -180,6 +180,32 @@ static void split_vertex_per_fan(const int vertex,
   }
 }
 
+static void split_vertex_loose_edges(const int vertex,

@@ -439,6 +465,23 @@ void split_edges(Mesh &mesh,
   add_new_vertices(mesh, new_to_old_verts_map);