Upstream `bevy_basisu_loader` / use `basisu_c_sys`

[beicause]

What problem does this solve or what need does it fill?

See also #23358
I believe basisu is still worth better support. In the snapshot tests of bevy_basisu_loader, a BC7 skybox with mipmaps is 32 MB (though it's not zstd compressed), while ASTC 8x8 and XUASTC 8x8 is only 8 MB and 5.3 MB. See also The-Three-Generations-of-GPU-Texture-Distribution
Pros and cons of different texture formats I can think of:

Format	Pros	Cons
Common image formats (png, webp)	Small file size. Easy to view, port, and edit	Slow loading. VRAM and Bandwidth aren't reduced
Native GPU formats	Fastest loading speed, no decoding. Reduced VRAM and bandwidth	Size on disk is large1. Requires separate versions for different platforms
Basis Universal formats	Small file size. Reduced VRAM and bandwidth. Portable	Slow loading2. Requires basis universal dependency3

What solution would you like?

Upstream bevy_basisu_loader.

What alternative(s) have you considered?

No.

Footnotes

1. ASTC with large block size is much smaller than BCn but isn't always available on desktop. There's also RDO which can reduce compressed size. ↩

2. No transcoding if basisu textures are standard ASTC and platform support it. Caching results to disk during installing/first loading is a solution, too. ↩

3. Needs C++ binding + wasm workaround. A pure Rust transcoder is a huge amount of work probably no one can finish it. ↩