It uses edge extrapolation code from Gimp's antialias plugin now,
which has advantage of giving symmetrical results, which makes it
possible to add two antialiased ID masks and have a constant 1.0
all over the frame. But has difference from the old implementation
because it uses 3x3 matrix only, which doesn't give so much smooth
looking edges. Perhaps it's not so bad, since if edges are really
need to be smooth one might use Blur node.

Another advantage is that the node is now nicely threaded.