stack: Allow promoting transient windows to their parent's layer

When a window that should be stacked above another one is placed in a lower
layer than the other window, we currently allow promoting it to the higher
layer when it has a "transient type". We should do the same when the window
is an actual transient of the other window.

This is particularly relevant for wayland windows, where types play a
much smaller role: Transient windows like non-modal dialogs (and since
commit 666bef7a, popup windows as well) currently end up underneath their
always-on-top parent.

https://gitlab.gnome.org/GNOME/mutter/issues/587


(cherry picked from commit 1c89fce30e)

src/core/stack.c

@@ -858,8 +858,11 @@ static void
 ensure_above (MetaWindow *above,
               MetaWindow *below)
 {
-  if (WINDOW_HAS_TRANSIENT_TYPE(above) &&
-      above->layer < below->layer)
+  gboolean is_transient;
+
+  is_transient = WINDOW_HAS_TRANSIENT_TYPE (above) ||
+                 above->transient_for == below;
+  if (is_transient && above->layer < below->layer)
     {
       meta_topic (META_DEBUG_STACK,
 		  "Promoting window %s from layer %u to %u due to contraint\n",