"Correct" overlay (uses 0-2,4-256 instead of 0-255)

function autismask(clip a, clip b, clip mask, int "y", int "u", int "v") {
y = default(y,3)
u = default(u,3)
v = default(v,3)
mt_lutxyz(a,b,mask,"x y x - z 2 > z 1 + z ? * 128 + 8 >> +",y=y,u=u,v=v)
}