Rebase the cursor after applying transactions

This approaches cursor rebasing from a different angle. Rather than
littering the codebase with cursor_rebase calls and using transaction
callbacks, this just runs cursor_rebase after applying every transaction
- but only if there's outputs connected, because otherwise it causes a
crash during shutdown.

There is one known case where we still need to call cursor_rebase
directly, and that's when running `seat seat0 cursor move ...`. This
command doesn't set anything as dirty so no transaction occurs.

sway/input/seat.c

@@ -1196,5 +1196,4 @@ void seat_consider_warp_to_focus(struct sway_seat *seat) {
 	} else {
 		cursor_warp_to_workspace(seat->cursor, focus->sway_workspace);
 	}
-	cursor_rebase(seat->cursor);
 }