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.
Yeah, but what's the interaction? Anything more than "setting the `buffer_size` here to anything greater than the `buffer_size` on the vector tiles won't have an effect"?