[3.x] SceneTreeFTI - Fix force_update flag for invisible nodes

[lawnjelly]

If the force_update flag remained set after the node was removed from the update lists, the node would never be updated in future. This could occur with hidden nodes that were moved after hiding.

Fixes #107142
Backport of #107175

Explanation

The SceneTreeFTI relies on node flags being in sync with nodes being on various update lists. This bug occurred for the forced frame update list, because the node flag was being cleared in the tree traversal, however invisible nodes were never having their flag cleared, and once the list was cleared the flag and the list became out of sync.

At further node movements, the logic determined incorrectly that the node was already on the update list.

We fix it by removing the flag manually on every node in the list rather than at traversal time.

Notes

• It's possible this can be done in a more optimal way at a later stage, but this shouldn't be bad.

[Ivorforce]

Approving as equivalent to #107175

[lawnjelly]

Thanks!