filter_sse2_intrinsics.c 
Functions in this file look at most 3 pixels (a,b,c) to predict the 4th (d).
They're positioned like this:
prev: c b
row: a d
The Sub filter predicts d=a, Avg d=(a+b)/2, and Paeth predicts d to be
whichever of a, b, or c is closest to p=a+bc.

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