Degenerate but valid ASTC configs: suboptimal CEM encodings

See: 

https://github.com/BinomialLLC/basis_universal/wiki/List-of-Available-Software-ASTC-Decoders#degenerate-but-valid-astc-configs

Updated: "ASTC GPU Texture Decoding: Software Decoders, Spec Issues, and ARM Errata"

ASTC's complexity has created an ecosystem where correctness is an ongoing research problem rather than a fully solved one. See:

https://github.com/BinomialLLC/basis_universal/wiki/List-of-Available-Software-ASTC-Decoders

.ASTC (the File Format): No Longer a Black Box

The basisu command line tool has a new option, -peek, which opens any standard ARM LDR/HDR .ASTC texture file, unpacks each block, and computes a bunch of statistics about the exact ASTC configurations the blocks used.

This is how we found out that Intel's ispc_texcomp's ASTC encoder is, for all practical purposes, broken.

https://github.com/BinomialLLC/basis_universal/wiki/Displaying-.astc-file-block-statistics-using-the-peek-command-line-option

ASTC Decoding: Software Decoders, Spec Issues, and ARM Errata

 See here:

https://github.com/BinomialLLC/basis_universal/wiki/List-of-Available-Software-ASTC-Decoders

ARM ASTC Decoding Errata

See here:

https://documentation-service.arm.com/static/67ca1a5ece2747241fced502

More general info on the real-world complexities of decoding ASTC are here:

https://github.com/BinomialLLC/basis_universal/wiki/List-of-Available-Software-ASTC-Decoders

Some things we've learned about GPU textures at planetary scale

1. ASTC is now the king: In billions of devices. Everything else=fallback, including BC7.

To us, BC7 is essentially a greatly simplified ASTC, but with some p-bits.

2. At multi-petabyte (planetary) scales: Supercompression bitrate=Matters enormously.

Notably, game developers (who have been using compressed textures the longest) don't work at scales this large, so the approaches and techniques they assume are correct or standard in this domain may not apply at all in extreme scales.

The tradeoffs game developers have made in the past are no longer aligned with modern hardware and network realities.

3. GPU Drivers=super sketchy.

This means for us: No driver usage, no compute. The largest vendors, who already deal with endless GPU driver bugs, don't want even more exposure in critical texture decompression/transcoding paths. If it fails for even ~.1% of customers in the wild, it's unusable.

4. All 14 ASTC block sizes are important in large scale deployment scenarios, not just 2 or 3.

This includes 12x12, which at 4k-8k is quite usable.

5. When mipmap and filtering compatible deblocking of the larger ASTC block sizes is trivial to do in a tiny pixel shader, it makes no sense not to deblock because the cost of not doing so is ~2x-8x more bitrate and bandwidth.

6. Unfortunately, LDR ASTC decoding actually isn't always bitwise exact. (BC7 wins for this, at least.)

The ASTC specification was so dense and complex even the vendors (including ARM itself!) couldn't get it right.

7. WASM SIMD isn't everywhere (or even when it is, some very big vendors won't allow it to be used or enabled), so that means we can't depend on SIMD. This means less searching, more math, and better algorithms in our encoders, or we can't ship.

8. Everything must be fuzzed. That means the obvious things like block decoders, all decompressors, etc. but it also includes encoders. Trust no data.

The Three Generations of GPU Texture Distribution

In the Basis Universal v2.0 wiki:

https://github.com/BinomialLLC/basis_universal/wiki/The-Three-Generations-of-GPU-Texture-Distribution