transients: Do no search for parent in lower layers

Bug: 15891

When raising a transient window we would automatically raise its parent
windows.

However, if the transient and parent are not placed on the same layer,
we might raise the parent from a lower layer.

To avoid that issue, limit the scope of search to the same or higher
layers.

Signed-off-by: Olivier Fourdan <fourdan@xfce.org>

src/transients.c

@@ -347,6 +347,11 @@ clientGetTransientFor (Client * c)
             continue;
         }
 
+        if (c->win_layer > c2->win_layer)
+        {
+            break;
+        }
+
         if (clientIsDirectTransient (c) && clientIsTransientFor (c, c2))
         {
             parents = g_list_append (parents, c2);