Add rendering capability

This adds the ability to render raster tiles from vector ones with the `avecado::render_vector_tile` function. Further, it adds the ability to fetch PBF tiles from vector tile sources supporting TileJSON files, see the `avecado::tilejson` function.
I'd probably write this

> Setting buffer_size will not cause additional tiles to be fetched and setting it to anything larger than the greatest buffer_size of the underlying vector tiles will not normally have an effect.

There's a couple of "edge" cases with that require the normally