UASTC benchmark

RGB PSNR over a 1,048,576 4x4 block compression torture test (random blocks from 81 test textures):
                                     
    Near-opt BC7 (BC7E slower):   41.743
    astcenc_thorough:             40.892
    UASTC (veryslow)->ASTC        40.373
    UASTC (veryslow)->BC7         39.965
    UASTC (slower)->ASTC          40.163
    UASTC (slower)->BC7:          39.782
    UASTC (default)->ASTC         39.372
    UASTC (default)->BC7:         39.171
    UASTC (faster)->ASTC          39.269
    UASTC (fastest)->ASTC         34.654
    UASTC (fastest)->BC7          34.554
    ispc_texcomp ASTC alpha_slow: 39.768
    stb_dxt BC1 HIGHQUAL:         32.479
    UASTC (slower)->BC1:          32.148
    UASTC (fastest)->BC1          32.256
    UASTC (slower)->ETC1:         30.956
    UASTC (fastest)->ETC1:        30.113
    UASTC (slower)->R11:          37.942

The 4096x4096 .PNG is here.

The EAC R11 format is R PSNR, and is included for comparison purposes.

Notice that the UASTC->BC1 quality actually increased when going from "slower" to "fastest" mode. This is because in "fastest" mode, almost all the blocks used UASTC mode 0, which is more compatible with BC1. (UASTC has 1-2 bits of BC1 hints per block that allow the UASTC block to be converted directly to BC1 blocks, skipping real-time encoding.)