USDZExporter: Export scene hierarchy

[mkeblx]

Fixed #31235.

Description

Preserve the scene hierarchy when exporting.

[Mugen87]

For testing: https://rawcdn.githack.com/mkeblx/three.js/usdz-exp-scngrph/editor/index.html

I'm not sure if this particular PR introduces this issue or #31442/#31400 but it does not happen in r178. If you open the editor and add a box, ambient and directional light and export to USDZ, you can't import the cube back anymore. It's missing after the import without reporting any errors in the browser console.

When using https://threejs.org/editor/, the round trip works as expected (meaning the cube is restored). I'll attach the exported scene below.

model.usdz.zip

[Mugen87]

I have investigated this closer and the regression is not caused by USDZExporter.

@mrdoob It seems c63f839 introduced the above issue.

You can use https://rawcdn.githack.com/mrdoob/three.js/c63f83942915af85128e1b57e87fc93cb3962a4c/editor/index.html for testing. Just add a box, export it to USDZ, empty the scene and import the model back. The box is not displayed.

[mrdoob]

I'll investigate this later today 🙏

[mkeblx]

Thanks for testing. I'll look at too, and will be adding more test coverage including the round trip soon.

[mrdoob]

Fixed! 173555e 🙏