bc7e.ispc integrated into bc7enc_rdo

bc7e.ispc is a very powerful/fast 8 mode encoder. It supports the entire BC7 format, unlike bc7enc's default encoder. It's 2-3x faster than ispc_texcomp at the same average quality. Now that it's been combined with bc7enc_rdo you can do optional RDO BC7 encoding using this encoder, assuming you can tolerate the slower encode times.

This is now checked into the bc7enc_rdo repo.

Command: bc7enc xmen_1024.png -u6 -U -z1.0 -zc4096 -zm

4.53 bits/texel (Deflate), 37.696 dB PSNR

BC7 mode histogram:
0: 3753
1: 15475
2: 1029
3: 6803
4: 985
5: 2173
6: 35318
7: 0