Fixes session lock surfaces not being able to switch focused output

[ErikReider]

Fixes: #7790

To reproduce the issue:

1. Start a nested sway session with two virtual outputs (swaymsg create_output WL-2)
2. Lock the session, preferably with a locker that shows if it receives focus like gtklock or my locker
3. Try switching focus to the non-focused input field on the other monitor

It would also be nice if this could be backported to v1.11

[ErikReider]

With the current code, when a seat wants to switch focus output, it forces the new surface to be the focus surface for every seat (in the focus_surface function). Let me know if that's not desired, and I'll fix it :)