v1.8.1

Changed

• Reading an 8-bit RGB or RGBA image as Gray8, Gray16 or gray+alpha (CopyPixelsTo, ToArray, direct decodes, region reads) is several times faster: the RGB-to-gray conversion now runs in integer SIMD (AVX2, 128-bit, scalar) instead of through float32 rows. A pixel whose exact value lies close to a rounding boundary is recomputed with the float expression, so every output byte is identical to before (checked for all 2^24 RGB triples and every alpha). A 4 MP-class page read as Gray8 now costs about the same as the Rgb24 read.