KTX2 for non-color data

First of all, I'm very excited to see the development on [KHR_texture_basisu](https://github.com/KhronosGroup/glTF/pull/1751).

We're looking at using KTX2 as an image format for storing non-color data, specifically uint16 and floating point data. We're working on a glTF extension for adding generic per-vertex and per-texel metadata to models and KTX2 seems like a good alternative to png/jpeg since data types are more clearly defined.

One example might be a per-texel "accuracy" that describes the the error in meters along a photogrammetry model compared to the ground truth. This can be stored in a KTX2 image using the `VK_FORMAT_R32_SFLOAT` format.

With this use case in mind there's still rationale to support baseline KTX2 in glTF. Maybe there's a chance of reviving https://github.com/KhronosGroup/glTF/pull/1612 or creating a new extension for this use case.

Also see https://github.com/CesiumGS/glTF/pull/1 for more details on the feature metadata extension.

Review the KHR_texture_basisu pull request 1751, the earlier pull request 1612, and the CesiumGS feature metadata pull request linked in the issue. Determine whether non-color KTX2 data belongs in baseline glTF support or a new extension; done means a decided scope and an agreed specification path.

Written by the indexing model from the issue text.