#417 Terra/Toposerver: Crossing UTM zone boundaries

[Curt Mills]

If we cross a zone boundary, Terraserver/Toposerver
will only give us a map for one side of the boundary.
Look in draw_geo_image_map() for zstr0/zstr1. If
they are different after the UTM conversion
functions, it means we are in this condition. It'd
be great if someday we could split the map fetch into
two map fetches when this condition is sensed, then
draw both of them, making sure to only draw up to the
UTM boundary for each.