Copyright 2004 Ken Greenebaum
Paeth predictive filter
nhoriz = abs(above – upperLeft);
nvert  = abs(left  – upperLeft);
ndiag  = abs(left + above – 2* upperLeft);
nif (horiz <= vert && horiz <= diag)
n return left;
nif (vert <= diag)
n return above;
nreturn upperLeft;