SceneTreeFTI - miscellaneous speedups

[lawnjelly]

This is just a small PR, in case there is a delay to merging #107369 before the next beta (that PR will change how we fast access children).

This fixes up the second case in _update_dirty_nodes() to use fast child access (that @Ivorforce noticed yesterday, I accidentally reverted it in #106244 😊 ).

Also accesses data.visible directly to avoid one thread guard.

Notes

• There are also numerous unneeded thread guards around e.g. get_transform() in the function, but that needs a PR of its own.
• This is purely performance, no change in behaviour.

[akien-mga]

Thanks!