BC7-compatible encoders

[mrRay]

to the best of my knowledge, libsquish doesn't support BC7 (nor do any of the forks i've looked at so far- somebody please correct me if i'm mistaken). support for the BC7 variant of hap will require use of another/a different texture compression library (potentially replacing libsquish if it supports BC1/4/5, if the metrics are good, and if it would result in a simplified codebase). this issue is meant to provide a place for assembling a list of all potential encoders for consideration.

requirements:

• support for enc/decoding of BPTC/BC7 UNORM textures
• bonus: support for enc/decoding of DXT1/BC1, DXT5/BC3, and Alpha RGTC1/BC4

[mrRay]

https://github.com/richgel999/bc7enc