F_avg_first
F_none, F_sub, F_none, F_avg_first, F_paeth_first
case F_avg_first : cur[k] = raw[k]; break;
CASE(F_avg_first) cur[k] = raw[k] + (cur[k-img_n] >> 1);
CASE(F_avg_first) cur[k] = raw[k] + (cur[k-out_n] >> 1);