Add support for KTX textures and KHR_texture_basisu?

It would be helpful if the glTF Validator supported KHR_texture_basisu and mimeType:"image/ktx2".

Currently the validator reports multiple Warnings and Infos related to KTX2, and thus does not list the dimensions of the bitmaps, among other useful infos.

The Stained Glass Lamp sample model includes an example with KTX2 textures:
https://github.com/KhronosGroup/glTF-Sample-Models/tree/master/2.0/StainedGlassLamp/glTF-KTX-BasisU
This model has the basisu extension listed in both extensionsUsed and extensionsRequired, because there are no fallback textures. (see https://github.com/KhronosGroup/glTF-Validator/issues/157)

Thanks!

No source file, test, or entry point is named in the issue. Start by inspecting how the validator handles texture mimeType values and extensions, then use the Stained Glass Lamp glTF-KTX-BasisU sample as the reproduction case. Done means KHR_texture_basisu and image/ktx2 no longer produce the reported warnings and infos, including missing bitmap dimensions; issue 157 provides related context.

Written by the indexing model from the issue text.