bc7enc_rdo now supports RDO for all BC1-7 texture formats

It now fully supports RDO BC1-7:

 https://github.com/richgel999/bc7enc_rdo

I've also been cleaning up the tool and tuning all of the defaults. Note that if you build with MSVC you get OpenMP, which results in significantly faster compression. Currently the Linux/OSX builds don't get OpenMP.

I decided to unify all RDO BC1-7 encoders so they use a single universal entropy reduction transform function in ert.cpp/.h. I have specialized RDO encoders for arrays BC1 and BC4 blocks (which I checked into the repo previously), which may perform better, but it was a lot more code to maintain. I removed them.