Rework passage creation on Lower Nubia and add map description by Sundiata, refs #5040.

Increase passage size so that elephants and siege engines can pass with higher likelihood.
Use the ElevationBlendingPainter from rP21182, so that the mountain elevation near the river is taken into account.
Smooth the passages even more using the SmoothingPainter, refs #5027.
Add missing SetProgress calls.