Test mapnik rendering backend

[brendan-ward]

As an alternative to Pillow.

Looks like we can feed in image data directly from python

In particular, we could leverage mapnik's png output options to also use lossy compression for smaller rendered images instead of relying on something like pngquant.

Downsides may be an increased dependency surface. Will need to benchmark performance, flexibility, and size of output files to see if it is worth it.

[brendan-ward]

There may also be some wins around using the antialiasing characteristics of mapnik to render features to a grayscale mask, then treat that as an approximation of weights for weighted pixel statistics.