Pure Rust encoding library

[ScanMountGoat]

Similar to #11, this will make compilation easier for some use cases. This will be more involved than decoding since intel_tex_rs uses the intel ispc compiler and is fairly complex. It may be better to make bindings to a smaller library first like https://github.com/elasota/ConvectionKernels and fuzz the Rust port against the original C++ code. It's also worth investigating the potential performance loss since intel_tex_rs makes heavy use of SIMD.

[ScanMountGoat]

Another option for all formats except bc2 and bc6 is https://github.com/richgel999/bc7enc_rdo.

[ScanMountGoat]

BC2 encoding can use the color block from BC3 encoding, so BC2 support isn't a hard requirement for an encoding library.

[Aerocatia]

We have been using https://github.com/jansol/texpresso (rust port of libsquish) in our custom Halo CE tools and have found it to have good quality results, however we are looking with something with bc7 support.