[glTF] KTX cubemaps support

[vorg]

Filament is using them, CesiumJS recently added support and there is discussion here and here on glTF forums.

Upsides:

• potentially better quality than realtime generated octahedral maps

Downsides:

• dependency on filament tooling currently
• matching dynamic generation would need to be implemented

[vorg]

Another reason to add ktx support https://www.npmjs.com/package/texture-compressor

CLI tool for texture compression using ASTC, ETC, PVRTC and S3TC in a KTX container. Easily compress for all hardware using sensible defaults but with the option to pass any custom flag directly to the tool.