Three more changes for the Mesh module:
(1) Bug fix for UV vertices ("sticky").  me->msticky was not being updated
   when vertices were added or deleted from the mesh.
(2) Vertex, edge and face .extend() methods accept zero-length sequences
   instead of throwing an exception.  (Note that not giving any argument
   is still an error).
(3) Edge and face .extend() methods ignore "invalid" edges or faces which
   have the same vertex twice instead of throwing an exception.  Cam and I
   argued about this for a while... :-)
Ken