Feature Request: Support KHR_materials_variants

[_herbst_]

glTF files can contain material variants; it would be great if Stager had a dropdown to select which of those variants should be displayed / rendered, similar to how the camera position can be selected.

More info here: https://www.khronos.org/blog/streamlining-3d-commerce-with-material-variant-support-in-gltf-assets

Sample here: https://modelviewer.dev/examples/scenegraph/#variants