GLTF: Preserve node visibility on import

[aaronfranke]

KHR_node_visibility is implemented by PR #93722 but is not yet ratified by Khronos. In the meantime, I tried to implement this in GDScript, but I found that it was impossible for 2 reasons:

• Visibility is not preserved when moving from ImporterMeshInstance3D to MeshInstance3D during import.
• Invisible nodes are discarded during export.

This PR extracts the fixes to these problems the first problem from PR #93722 to allow implementing KHR_node_visibility in GDScript. EDIT: For import only.

The fix for not discarding invisible nodes may be controversial, especially since due to the lack of the user-facing settings in PR #93722, there is no exposed setting to customize this. Still, I think this is a good change. I would exclude this part when cherry-picking to 4.3 though. Or, I can remove it if I really must, and at least we can fix importing.

[fire]

I would move the not discarding invisible nodes to the node visibility pr unless you want to do the full forward compatibility toggle dance. It's up to you.

We can do it now or later.

[aaronfranke]

@fire Updated the PR to remove the "not discarding invisible nodes" part.

[fire]

We discussed:

The problem with adding a user-facing option is that the three options should be "Include & Required," "Include & Optional," and "Exclude."

Without KHR_node_visibility, the first two would behave the same.

This causes an option that would change meaning in a future release.

Another option is that I could add in every bit of code except the reading/writing to GLTF part, making it trivially easy for people to implement KHR_node_visibility in GDScript, but it would be mostly unused code out of the box.

I do not favour this because it doesn't default on, so any bugs will be hidden for maybe a year.

[fire]

The artifact upload failed, so feel free to press the redo failed jobs button.

[aaronfranke]

The cherrypick was not trivial so I removed the lables and opened PRs #98877, #98878, and #98879.

[Repiteo]

Thanks!