Low bitrate RDO BC7 with lzham_devel

RDO BC7+Deflate could also be described as "BC7 encoding with Deflate in-loop".

Using the lzham_codec_devel repo (which is now perfectly stable, I just haven't updated the readme kinda on purpose), this mode 1+6 RDO BC7 .DDS file compressed to 2.87 bits/texel. LZMA gets 2.74 bits/texel. 

Around 10% of the blocks use mode 1, the rest mode 6. I need to add a LZMA/LZHAM model to bc7enc_rdo, which should be fairly easy (add len2 matches, add rep model, larger dictionary - and then let the optimal parsers in lzham/lzma figure it out).

Commands:

bc7enc -zc32768 -u4 -o xmen_1024.png -z6.0

lzhamtest_x64.exe -x16 -h4 -e -o c xmen_1024.dds 1.lzham

There are some issues with this encoding, but it's great progress.