FTI - `global_transform_interpolated()` on demand for invisible nodes

[lawnjelly]

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Fixes #107235 Forward port of #107307

Explanation

For visible nodes, the interpolated transform is already cached and calculated in SceneTreeFTI. However, invisible nodes do not calculate interpolated transforms (in order to save processing), thus getting their interpolated transform directly is not possible.

Instead we add mechanism to calculate this interpolated transform on the fly. This will be more expensive than for visible nodes, but can be handy in some situations.

Notes

• The limitation of _is_vi_visible() to VisualInstances is getting a little tiresome now, so I have a follow up PR (#107330) to change the calculation and caching of is_visible_in_tree(), but that should be considered as a separate issue, and this PR will run fine without it.