XUBC7/XBC7: Trellis quantization added

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.…