This way it's clear what's the input and what's the output of the
computation.
All map generation scripts should reman working. They are adopted in a
future commit.
`Engine.SetProgress` and `Engine.ExportMap` can be removed in a future
commit.
Comments by: @marder, @sera, @Stan
Differential Revision: https://code.wildfiregames.com/D5220
This was SVN commit r28093.