OptionalonCalled once after parsing, before the Scene is returned to the asset pipeline. Use to read metadata that isn't represented in the three.js Scene — Blender's embedded thumbnail, raw DNA struct data, library/kinematics references, etc.
The callback's return value is ignored. Don't put data here onto userData (the import/export
pipeline will round-trip it as JSON — large typed arrays like the thumbnail will balloon
exports or fail serialization).
Loader options consumed by
BlendLoadPlugin. Pass viaviewer.load(url, options).