Among other things, this gave HttT S17 (Sceptre of Fire) a 50% chance
of crashing at scenario start.
(cherry-picked from commit 7d0d9e19a5fce052e35f866a1d83584dcd8f20bb)
actual bugs found:
* backwards_compatibility.lua (undeclared global "helper")
* core.lua (use of undeclared global "helper")
* wml_tags.transform_unit had wrong code to deal with recall_cost
* wrong variable name in cave_map_generator
This accomplishes two main things:
- Creates a mapgen_helper.lua module containing functions useful for Lua map generators
- Paves the way for allowing the cave generator to produce scenarios and accept user configuration
