Rate distortion performance of Basis ETC1 RDO+LZMA on the Kodak test set

At 3 quality levels, using REC709 perceptual colorspace metrics. This compares plain ETC1 (with no lossless compression), basislib highest quality ETC1+LZMA, and basislib RDO+LZMA.

"S" = selectors, "E" = endpoints.

crunch-style adaptive endpoint quantization at the block/subblock level is supported, but not at the macroblock (2x2 block) level yet. Also, the KTX writer backend is greedy, meaning it doesn't try to choose the best combination of selectors+endpoints that result in the least amount of compressed bits output by LZMA (or LZHAM). The lack of both features hurts compression. I have several other improvements to both quality and bitrate coming, but this is a good milestone.

With a few more quality levels: