Support specifying map boundary and warn for giant maps

[gdude2002]

This means that users will be able to specify just the area they want to map, and thus orphaned tiles outside of that area won't result in a huge map.

[gdude2002]

It'd also be nice to detect orphaned tiles somehow, but I have no idea where I'd begin on that one.

[caillou15]

For giant map, it can be useful to change the scale.