Allow 16-pixel loopfilter functions.

We could (future) allow 32-pixel (for AVX2) also, but that's not
necessary ATM. It'd also be nice to allow to mix 4-wd and 8-wd
functions in a single call; this would give speed-ups in SIMD
versions, although the C might be slightly slower. For now, I'll
leave that as a future TODO.