Avoid incorrect computing anchor of Control node when reset on save with saving flag

[TokageItLab]

Fixes #90079. Alternative of #91493.

#91493 seems to have some effect on the scene and cannot pass the editor test. Possibly the test is not correct (like process timing problem?) but I don't know the details...

It seems a bit of a stopgap, but this PR adds the saving flag and provides a narrower fix focused only on save time.

[KoBeWi]

This makes RESET animation have no effects on Control nodes. You could at least add is_inside_tree() check (in addition to saving), to make the position reset when saving the current scene. RESET would still have no effect when saving inactive scene, but at least it wouldn't break layouts.

[TokageItLab]

@KoBeWi Thanks and indeed. I added !is_inside_tree() check. I assume this workaround will now only work if the scene is saved in a background tab.

[KoBeWi]

It's a hack, but at least it fixes the issue.

[akien-mga]

Thanks!