KTX2 ecosystem requirements

This is a meta issue, tracking tasks required for KTX2 (and KHR_texture_basisu) to be considered production-ready. Related issues are grouped in the [compressed textures milestone](https://github.com/KhronosGroup/glTF/milestone/3).

**KTX2 material authoring:**

- [ ] Complete pipeline for producing glTF files with KTX2 embedded, given glTF files with PNG/JPG embedded.
- [x] [gltfpack](https://github.com/zeux/meshoptimizer/tree/master/gltf) converts textures to ETC1S or UASTC
- [x] [libktx](https://github.com/KhronosGroup/KTX-Software/tree/ktx2) will be a useful part of this pipeline, but doesn't do the full glTF->glTF conversion itself.
- [x] [glTF-Transform](https://gltf-transform.donmccurdy.com/extensions.html#khr_texture_basisu-experimental) converts textures to ETC1S or UASTC
- [ ] [glTF-Pipeline](https://github.com/AnalyticalGraphicsInc/gltf-pipeline) can roundtrip files using KTX, converting textures in progress.
- [ ] Test pipeline against many glTF models, compare results in viewers (see below), and write up best practices if necessary.
- [ ] (Optional) Direct export to glTF+KTX2 from DCC tools (Maya, Blender, 3DS Max, ...)

**KTX2 material clients:**

- [x] 2+ viewers/engines for testing glTF assets containing KTX2 material textures.
- [x] [three.js#18490](https://github.com/mrdoob/three.js/pull/18490)
- [x] babylonjs
- [ ] glTF-Sample-Viewer?
- [ ] 1+ production-ready, lightweight transcoder.
- [ ] [KTX-Software](https://github.com/KhronosGroup/KTX-Software/) provides a transcoder. Some remaining tasks surrounding where the final transcoder should be maintained, and minimizing the size of the transcoder.
- [ ] [AssemblyScript/WASM transcoder](https://github.com/KhronosGroup/Universal-Texture-Transcoders)

**IBL:**

To be determined.

Start with the compressed textures milestone and review the unchecked tasks under KTX2 material authoring, material clients, and IBL. Coordinate with the linked glTF-Pipeline, glTF-Sample-Viewer, KTX-Software, and Universal-Texture-Transcoders work; done means the listed ecosystem requirements are resolved and production-readiness guidance is established.

Written by the indexing model from the issue text.