Fix Container::pending_sort tracking

When Container::queue_sort() is called, pending_sort is set to true to indicate when a call to _sort_children() is queued, to avoid queueing multiple calls. Container::_sort_children() sets pending_sort back to false when finished, but did not do this when the container was not inside the tree.  This would allow cases where queue_sort() could be called just before removing from the tree, causing _sort_children() to never reset pending_sort, preventing any future queue_sort() calls from queueing again.

One case where this happened was with the "Saving Scene" progress bar in the editor - when saving for the first time (or the first time the progress bar popup otherwise appeared in the editor), _sort_children() would be called successfully.  After the progress bar popup was hidden, then shown again on future saves, _sort_children() would not be called again, resulting in the progress bar not taking up as much space as it should.

This issue used to be avoided by setting pending_sort to false immediately on NOTIFICATION_ENTER_TREE - however, this would allow multiple calls to be queued at the same time when entering the tree (godotengine#92644).  The multiple calls was fixed recently by removing this assignment, but this also made possible the case where pending_sort is never reset.

This change sets pending_sort back to false in _sort_children() whether or not it's in the tree.  Since this is done in a deferred call, it should still avoid the previous issue of multiple calls being queued at once on entering the tree.

Fixes godotengine#92971

scene/gui/container.cpp

@@ -80,6 +80,7 @@ void Container::remove_child_notify(Node *p_child) {
 
 void Container::_sort_children() {
 	if (!is_inside_tree()) {
+		pending_sort = false;
 		return;
 	}