CPU BC1 Encoding Pareto Frontier

rgbcx.h now defines the BC1 Pareto Frontier for high quality CPU BC1 encoding (i.e. it's stronger than all other available practical high quality CPU encoders for both performance and quality):

Data:



I didn't include AMD Compressonator's encoder because in previous benchmarks (conducted by others) it was beaten by a weaker version of rgbcx.h for both perf. and quality.

The overall CPU BC1 Pareto frontier is defined by ispc_texcomp (at low quality: ~33.1 dB) and rgbcx for any higher quality level. We're going to need SIMD to compete against ispc_texcomp BC1 (a weak stb_dxt clone), which is my next major goal.

To get rgbcx to compete against icbc for max. quality I had to add prioritized cluster fit support for 3-color blocks (not just 4).

It's possible to permit rgbcx to go to even higher quality levels by enlarging the total ordering tables. They're currently limited to 32 entries per total ordering.

I think rgbcx.h's max quality is slightly higher than icbc's HQ mode because prioritized cluster fit can afford to do optimal rounding and evaluate accurate MSE errors in every trial. Regular cluster fit can't afford to do so because it has to evaluate so many total orderings.

Links:
rgbcx: https://github.com/richgel999/bc7enc
libsquish: https://github.com/richgel999/libsquish
icbc: https://github.com/castano/icbc/blob/master/icbc.h

stb_dxt: https://github.com/nothings/stb/blob/master/stb_dxt.h