Fix cursor events on screen lock/unlock

After the scene graph refactor, node_at_coords no longer return the
lock surface after the screen is locked, thereby cursor_rebase_all in
handle_surface_map won't work.

This commit make node_at_coords return lock surface when locked.

I also found that the cursor would disappear after screen is unlocked,
thereby adding a cursor_rebase_all in handle_unlock.

sway/input/cursor.c

@@ -63,6 +63,11 @@ struct sway_node *node_at_coords(
 		}
 	}
 
+	if (server.session_lock.lock) {
+		*surface = server.session_lock.lock->focused;
+		return NULL;
+	}
+
 	if (scene_node) {
 		// determine what wlr_surface we clicked on
 		if (scene_node->type == WLR_SCENE_NODE_BUFFER) {

sway/lock.c

@@ -234,6 +234,7 @@ static void handle_unlock(struct wl_listener *listener, void *data) {
 		struct sway_output *output = root->outputs->items[i];
 		arrange_layers(output);
 	}
+	cursor_rebase_all();
 }
 
 static void handle_abandon(struct wl_listener *listener, void *data) {