Now with fixed zoom level for the hunting.

This is a very basic idea, the downside is that you basically cannot
"fix" the tilehunt overlay to a specific zoom level. It might be nice to
get a second renderer that renders the tile hunt for a fixed zoom, and
then adjusts the output tiles accordingly (then it also takes more space
again though).

This is in prepration for the tilehunt mode, where we want to render
tiles differently.

Since we want to support SQLite at some point, it makes sense to have
the exact storage method abstracted away.

This is another pretty CPU bound task (parsing the XML files), so it
makes sense to parallelize. We already have rayon, we already have the
setting to control parallelism, so let's use it and make hittekaart
fast!