XBC7/XUBC7 (supercompressed weight grid residual DCT+endpoint DPCM+Zstd) prototype is done

The next step is integration and connecting the BC7->ASTC LDR 4x4, ETC1, etc. transcoders into place in the Basis Universal library. Bitrate and quality at low Q's (Q=1/100) are exceptional vs. XUASTC LDR 4x4 (~1.5-3.0 bpp), and at Q=100 it's lossless in BC7 space at up to ~5.6 bpp or less depending on the content. Q=1 is totally usable, unlike XUASTC LDR where it falls apart because it's stuck…