stack: always update window position at transition start

Sometimes on wayland we're seeing a frame draw before the first tick
callback, where the window position has not yet been updated.

With this change, the window is properly positioned as soon as
gtk_stack_start_transition returns.

https://bugzilla.gnome.org/show_bug.cgi?id=766405

gtk/gtkstack.c

@@ -1029,8 +1029,9 @@ gtk_stack_start_transition (GtkStack               *stack,
       gtk_stack_unschedule_ticks (stack);
       priv->active_transition_type = GTK_STACK_TRANSITION_TYPE_NONE;
       gtk_progress_tracker_finish (&priv->tracker);
-      gtk_stack_progress_updated (GTK_STACK (widget));
     }
+
+  gtk_stack_progress_updated (GTK_STACK (widget));
 }
 
 static void