Add BC7 encoder with RDO

[MarkCallow]

We want to add BC7 encoding to the library with RDO (Rate-Distortion Optimization) to take better advantage of zstd supercompression. Rich Geldreich's open source Apache 2.0-licensed encoder at https://github.com/richgel999/bc7enc_rdo is a good candidate. The addition should follow the same model as the ASTC encoder.