[Feature] Texture compression

[kanerogers]

Background

At present we're texture bound for most things in Hotham. Let's fix that.

TODO

• Determine best formats
• Implement KTX2 support for glTF files
• Implement for test assets
• Profile

[kanerogers]

Notes:

https://github.com/KhronosGroup/3D-Formats-Guidelines/blob/main/KTXArtistGuide.md

[kanerogers]

From https://developer.qualcomm.com/sites/default/files/docs/adreno-gpu/developer-guide/gpu/best_practices_texture.html

The following strategy is recommended for selecting a texture compression format:

1. Use ASTC compression if it is available.
2. Otherwise, use ETC2 compression if available.
3. Otherwise, select the compression format as follows:
   a. ATC if using alpha
   b. ETC if not using alpha

Let's give that a go!

[kanerogers]

Right, a bit of fuckery is going to be required here:

• We can't really support basisu because Compressed texture support bevyengine/bevy#3608 (comment).
• So instead we'll do as @cwfitzgerald suggests and just use target specific ktx2 files
• Since we're insistent on not ever reading from the disc, this does mean that we'll have to use two "pre-compressed" glTF files, one for desktop and one for mobile

[kanerogers]

This is starting to look really, really good!

Remaining steps:

• Benchmark current implementation
• Finish off texture swizzling