Lower Nubia random map, refs #5040.

Composes the NASA heightmap with the NASA composite photography to reduce the ahistorical Lake Nasser and avoid having huge lakes in the valleys to the west of the Nile,
as the heightmap does not correlate with water distribution in this arid climate at all.
I.e. places riverbed elevation where composite and heightmap intersect, elsewhere uses the elevation model by NASA.

Bounding box chosen by Hannibal Barca
Geographic and biome consulting by Sundiata