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 126.96.36.199 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.
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