native-window-placement: Adjust to 42 changes

gnome-shell's new screenshot UI reuses the overview's window
picker layout, but its window previews don't give access to
the underlying MetaWindow.

Adjust to that by using the boundingBox property instead, which
is all we really need from the window anyway.

https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/issues/399

Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell-extensions/-/merge_requests/229>