Implement aastep

aastep was described in the docs, but I never quite got around to implementing it.

The docs originally described several variants that took an optional filter name, but I have removed this under the assumption that nobody really wants or needs it, and it unnecessarily complicates the implementation.  I polled several of our shader writers about whether then used that feature in some of the other widely-used commercial renderers I'd written, and for the most part they didn't recall that it existed, and possibly never knew. So I feel safe removing it from the docs, we can always add it later if it becomes imprtant.

Along the way, I found an error with filterwidth -- it wasn't marked in the table as requiring derivs, that could really mess it up.