Support ASTC formats

[RunDevelopment]

ASTC is the leap after BC6/7 and should be supported. The main problem is that Microsoft hasn't updated the DX spec and the official docs for DXGIFormat yet, so we'll have to rely on what others are doing.

Encoding looks to be even more difficult than BC7, so just supporting decoding should be the first step.

NVTT can serve as a reference implementation as it supports both reading and writing LDR ASTC formats. DirectXTex does not support ASTC at all.

References:

• https://github.com/ARM-software/astc-encoder/blob/main/Docs/FormatOverview.md
• https://github.com/ARM-software/astc-encoder
• https://forums.developer.nvidia.com/t/nv-tt-exporter-astc-compression/122477/2

[RunDevelopment]

There's a 100% safe Rust implementation of an ASTC decoder. Maybe astc-decode is suitable? License might be an issue.