Benchmark against Pillow-simd

[doug-wade]

Pillow SIMD purports to be a 2x boost in performance over Pillow for resizing, which would be a significant performance boon for us.

[eschwim]

Done. 2x might be a little overstatement in our case. But we did get an extra 10% render speed out of Dirpy with Pillow-SIMD with no visible drawbacks, so it's definitely a Good Thing. Adding it as a recommendation in the benchmarks and README docs.