Add a max_width/height argument to angular_ipred_fn

This is used in z2 to limit the number of pixels over which the filter is applied, as per "numPx" in 7.11.2.4 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.