method=0: Don't evaluate any predictor

and apply Paeth predictor (predictor#11) for the low effort (m=0) mode.

For 1000 image PNG corpus (m=0), this change yields speedup of 25% at lower quality
range and about 10% for higher quality range.

Change-Id: I0f036b8ffe45c241e63a067cbf01527b13d8de93