Initialize window->user_rect in the initial placement

The window positioning is delayed in idle_move_resize() in case the application
resizes/maximizes its window quickly after its creation. The delayed
positioning uses window->user_rect because of bug 426519 comment 3 (see
meta_window_move_resize_now()).

user_rect was not set in the initial positioning, causing the delayed
positioning unable to know which monitor we use for this window. As a
consequence, the window could jump spontaneously from one monitor to another.

With this patch, the window does not jump anymore.

https://bugzilla.gnome.org/show_bug.cgi?id=556696

src/core/constraints.c

@@ -571,6 +571,11 @@ place_window_if_needed(MetaWindow     *window,
                        .083 * info->work_area_monitor.height;
             }
 
+          /* idle_move_resize() uses the user_rect, so make sure it uses the
+           * placed coordinates (bug #556696).
+           */
+          window->user_rect = info->current;
+
           if (window->maximize_horizontally_after_placement ||
               window->maximize_vertically_after_placement)
             meta_window_maximize_internal (window,