F/allow custom importer

[ketourneau]

In my project I would like to override default importer to use GLTFast.Newtonsoft.GltfImport

protected override GltfImportBase GetDefaultImporter (
        IDownloadProvider downloadProvider = null,
        IDeferAgent deferAgent = null,
        IMaterialGenerator materialGenerator = null,
        ICodeLogger logger = null
    )
    {
        return new GLTFast.Newtonsoft.GltfImport(downloadProvider, deferAgent, materialGenerator, logger);
    }

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