Improve glTF plugin compatibility with newer ScriptedImporterAttribute syntax

[bitinn]

Is your feature request related to a problem? Please describe.

I am testing interoperability across various glTF plugins for Unity, and run into an issue between multiple ScriptedImporter declaring itself as default importer for glb or gltf file type.

Unity will disable all importers for glb and gltf files upon seeing this, and an error will be logged:

Multiple scripted importers are targeting the extension 'glb' and have all been rejected: ...

Describe the solution you'd like

This fork of UnityGLTF is the only plugin that doesn't conflict with other glTF importer via the use of overrideExts in ScriptedImporterAttribute (an added feature since 2020.2).

https://docs.unity3d.com/2022.2/Documentation/ScriptReference/AssetImporters.ScriptedImporterAttribute.html
https://github.com/prefrontalcortex/UnityGLTF/blob/master/UnityGLTF/Assets/UnityGLTF/Editor/Scripts/GLTFImporter.cs#L24

Describe alternatives you've considered

N/a

Additional context

I have tested this change to work:

Note: there is however a current problem that cause Importer dropdown to not show up, we are waiting for Unity to address this issue or provide workaround:

https://forum.unity.com/threads/scriptedimporterattribute-handling-blocks-multiple-importers-for-the-same-file-type.1248427/
prefrontalcortex/UnityGLTF#37

[bitinn]

Side note: these are the compatibility of GLB file across various popular glTF plugins.

[ousttrue]

Thank you for reporting.
overrideExts looks good.

The orientation of the model is affected when converting gltf (right handed) and unity (left handed).
UniGLTF has historically used z-axis inversion for both import and export.
UniGLTF can change the axis to invert.

• import https://vrm.dev/en/gltf/glb_import#reverseaxis
• export https://vrm.dev/en/gltf/glb_export.html

The model indices exported by glTFast appear to be failing.
Perhaps UniGLTF does not have an implementation of the unsigned byte type.

[ousttrue]

overrideExts will be in v0.100.0