~20% sRGB to scRGB speedup for common 3 and 4 band images
~20% sRGB to scRGB speedup for common 3 and 4 band images

lovell/libvips,jcupitt/libvips,jcupitt/libvips,lovell/libvips,jcupitt/libvips,lovell/libvips,lovell/libvips,jcupitt/libvips