Ensure that seat->focused_layer is cleared on unmap

The current seat may have changed between the last focus change and
this unmap, so we need to scan through all seats to find our layer.

sway/desktop/layer_shell.c

@@ -301,9 +301,11 @@ static void unmap(struct sway_layer_surface *sway_layer) {
 	output_damage_surface(output, sway_layer->geo.x, sway_layer->geo.y,
 		sway_layer->layer_surface->surface, true);
 
-	struct sway_seat *seat = input_manager_current_seat();
-	if (seat->focused_layer == sway_layer->layer_surface) {
-		seat_set_focus_layer(seat, NULL);
+	struct sway_seat *seat;
+	wl_list_for_each(seat, &server.input->seats, link) {
+		if (seat->focused_layer == sway_layer->layer_surface) {
+			seat_set_focus_layer(seat, NULL);
+		}
 	}
 
 	cursor_rebase_all();