Fix a crash when unlocking or unconfining a pointer

In GNOME (but not in Weston), if a window loses focus, the client first receives
the focus event, then the unlock/unconfine event. This causes toytoolkit to
dereference a NULL window when unlocking or unconfining the pointer.

To repro:
- Run weston-confine
- Click the window
- Alt-Tab away from it

Result:

[1606837.869] wl_keyboard@19.modifiers(63944, 524352, 0, 0, 0)
[1606837.926] wl_keyboard@19.leave(63945, wl_surface@15)
[1606837.945] wl_pointer@18.leave(63946, wl_surface@15)
[1606837.956] wl_pointer@18.frame()
[1606837.961] zwp_confined_pointer_v1@26.unconfined()
Segmentation fault (core dumped)

To fix this, get the input from the window instead of the other way around.

Signed-off-by: Dima Ryazanov <dima@gmail.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.co.uk>