This is used in z2 to limit the number of pixels over which the
filter is applied, as per "numPx" in 188.8.131.52 point 4 in the AV1
specification. This only applies to z2, because in z1/3, the edge
filter is (incomprehensibly) lengtened by the opposite side's edge
length, which undoes the limit on the filter length (like a bug
undoing another bug).
I admit the code is getting rather complex, so we may want to
redesign this to make writing SIMD easier.