use sharp instead of mapnik

[riastrad]

Currently, in order to do the final blending/cropping of Static Tiles, this library uses node-mapnik. This is a pretty beefy dependency to include solely for that purposes, it would potentially be better to use something written primarily for the purpose. After a bit of research, I would be happy with porting it to sharp, which has comparable features and is a bit lighter weight.

cc/ @tristen

[riastrad]

I had a little bit of slack in my schedule so I started messing around with Sharp over in quilt@riastrad/looking-sharp.

Unfortunately, it wasn't a super straightforward implementation because sharp's composite operation does not allow you to use top & left placement values that are outside the bounds of the final image you are generating (like we currently do with mapnik).

Ultimately, this would require a bigger refactor of quilt's image compositing logic than I'm willing to commit to at the moment.