F_paeth_first
F_none, F_sub, F_none, F_avg_first, F_paeth_first
case F_paeth_first: cur[k] = raw[k]; break;
CASE(F_paeth_first) cur[k] = (uint8) (raw[k] + paeth(cur[k-img_n],0,0));
CASE(F_paeth_first) cur[k] = (uint8) (raw[k] + paeth(cur[k-out_n],0,0));