Baked-in Slippy Tile server

[ivangayton]

It might be nice to allow for direct serving of tiles from the server running Tilehuria-flask.

• The tiles are already downloaded onto the server. It would be handy to have them instantly available for a mapathon.
• This would be particularly useful if someone is planning a mapathon and preps a POSM-style server beforehand (or even in a high-bandwidth country prior to traveling to a low-bandwidth place). In some cases serving tiles on a LAN might be preferable to distributing MBTiles on removable media.