Revert "csd: Drop the GTK_FRAME_EXTENTS requirement"

This reverts commit fb9a6bb6d8.

In a recent test, I've found that Xfce and Mate now support
this, so they will not be affected by this requirement. And
adding the check back will solve the 'client-side shadow'
problem in KDE.

gtk/gtkwindow.c

@@ -3955,6 +3955,9 @@ gtk_window_supports_csd (GtkWindow *window)
       if (!gdk_screen_is_composited (screen))
         return FALSE;
 
+      if (!gdk_x11_screen_supports_net_wm_hint (screen, gdk_atom_intern_static_string ("_GTK_FRAME_EXTENTS")))
+        return FALSE;
+
       /* We need a visual with alpha */
       visual = gdk_screen_get_rgba_visual (screen);
       if (!visual)