Add and use capability of defined area by min/max lat/lons for map types.

Various tile servers have differing coverage extents by area.
Thus don't need to generate download requests that we know are going to fail.
The area levels can be customized for new maps.

NB this only caters for a simple rectangular bounding box, rather then bounding polygon.
All built in maps have world wide coverage.
Only the extension configured map CalTopo is effected limited area to the US.