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