reload_transient_for: avoid xtransient_for loops

Don't set a window's xtransient_for if it would create a loop. Since
this is the only place we ever set xtransient_for, we can therefore
assume everywhere else that it does not loop.

https://bugzilla.gnome.org/show_bug.cgi?id=647712

src/core/display.c

@@ -5194,7 +5194,7 @@ prefs_changed_callback (MetaPreference pref,
           MetaWindow *parent = meta_window_get_transient_for (w);
           meta_window_recalc_features (w);
 
-          if (w->type == META_WINDOW_MODAL_DIALOG && parent && parent != w)
+          if (w->type == META_WINDOW_MODAL_DIALOG && parent)
             {
               int x, y;
               /* Forcing a call to move_resize() does two things: first, it handles