Implement KHR_materials_variants

[ikeough]

The location function on Hypar supports viewing map and satellite textures on tiles, but switching between textures requires a re-execution of the function. It would be great if we had a way of embedding both textures in the gltf then switching between them in the UI without a re-execution.

Describe the solution you'd like The KHR_materials_variants extension supports this. Serializing to glTF results in a KHR_materials_variants extension block. An API is provided in Elements that enables material variants. This would also enable users to supply multiple materials as they build a configurator for their product. A multi-material input would internally create a material variant, and the returning glTF would have material variants.

Describe alternatives you've considered We currently re-execute to change materials and it is slow. We could add client-side functionality in Hypar that enable the application of materials, but we wouldn't be able to run Hypar workflows completely on the server.