a fallback (or default if need be) generator in case no layer specified
generator is found. Added sample/ basis mod to demonstrate registration
of map generators. Rewrote Proxima Testauri map generator to conform to
multi_map, simple map generator is fallback in demo. Testauri now uses
a single noise which increases generation time but reduces memory usage.
To try and reduce generation time, it keeps track of the last layer used
and based on that will not reinitialize unless the layer changes.
evrooije