Editor: .gltf assets with utf8 characters in filenames can't be loaded

While tackling various internationalization issues with glTF assets I made a few new test assets:
- https://github.com/KhronosGroup/glTF-Sample-Assets/pull/157

Almost none of them load at all when either the folder or the zip are dropped into [modelviewer.dev/editor](https://modelviewer.dev/editor). Most of them load correctly in other glTF viewers, so something about path handling in the editor might not be quite correct.