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