The code here was wrong in every way: it only updated the shape if all the
borders changed. It never saved new last_borders even if it *had* changed,
and the bounding rectangle's x and y positions were still important otherwise.
This had user-visible impact when doing simple things like changing the
border_width. It would short-circuit here and due to the above incorrectness,
weirdness could happen where windows would be cut off and so on.
https://bugzilla.gnome.org/show_bug.cgi?id=656334
46415bb248
