Average rate-distortion curves for bc7enc_rdo

bc7enc_rdo is now a library that's utilized by the command line tool, which is far simpler now. This makes it trivial to call multiple times to generate large .CSV files.

If you can only choose one set of settings for bc7enc_rdo, choose "-zn -U -u6". (I've set the default BC7 encoding level to 6, not sure that's checked in yet.) I'll be making bc7e.ispc the new default on my next checkin - it's clearly better.

All other settings were the tool's defaults (linear metrics, window size=128 bytes).

My intuition was to limit the BC7 modes, bias the modes/weights/p-bits/etc. That works and is super fast to encode (if you can't afford any RDO post-processing at all), but the end result is lower quality across most of the usable range of bitrates. Just use bc7e.ispc.

128 vs. 1024 window size:

One block window size, one match per block: