Set base_path and filename during GLTF export when writing to a file

[aaronfranke]

This PR sets base_path and filename during the export process, and exposes filename. Previously only base_path was exposed and it was only used during the import process.

🚲🏚️: I used file_name for consistency with most of the engine's file_path, but as for just file names and not paths filename is twice as common as file_name. Personally I prefer file_name.

[fire]

I rather not add to the inconsistency. Any suggestions? @YuriSizov

[lyuma]

Approved in terms of technical review.

I'll leave bikeshedding of file_name up to others

[YuriSizov]

In the public API filename is used multiple times and file_name is used never (it is only ever found in docs in a code sample of DirAccess as a local variable name). So for consistency it should be filename.

[YuriSizov]

Looks fine to me. Needs a rebase though.

Also, and this is not new to this PR, but shouldn't getters be const for GLTFState members? (Note that changing that would require adding compatibility methods for the GDExtension API.)

[aaronfranke]

@YuriSizov Rebased and re-tested. Thanks for the catch with GLTFState property constness. Since filename is being added in this PR and copyright is not in Godot 4.1, I have changed those to be const. But base_path and the other properties were already exposed in Godot 4.1 so those will have to wait for the future to be changed to const.

[akien-mga]

But base_path and the other properties were already exposed in Godot 4.1 so those will have to wait for the future to be changed to const.

They could be changed now, you then just need to add compatibility methods so that existing compiled GDExtensions can find the old method hash and keep using it. See examples in #80168.

[akien-mga]

Thanks!