XUBC7/XBC7: Trellis quantization added (opens in new tab)

We just added Trellis quantization to XUBC7 \(supercompressed universal BC7\) via AC truncation, as long as the PSNR drop is within a configurable channel-weighted PSNR window, and the PSNR doesn't fall below a lower limit\. This helps at higher Q \(DCT quality\) levels\. Trellis quant is another video method we've ported successfully into GPU texture supercompression\. This new encoder also supports several forms of block-level RDO \(all optionally\) for surprising gains\.