window: Include window itself in transient loop check

While it doesn't make sense to set a window as transient to
itself, our existing check whether making a window transient
doesn't cover it, so it's still possible to create an infinite
loop.

https://bugzilla.gnome.org/show_bug.cgi?id=783502

src/core/window.c

@@ -7508,7 +7508,7 @@ check_transient_for_loop (MetaWindow *window,
 {
   while (parent)
     {
-      if (parent->transient_for == window)
+      if (parent == window)
           return TRUE;
       parent = parent->transient_for;
     }