Properly remove SAO when worldedges are overtaken (#5889)

Properly remove SAO when worldedges are overtaken (#5889)

* LuaEntitySAO: Remove beyond outermost mapchunk edges

Based on a commit by, and with help from, nerzhul.
Add 2 functions to class Mapgen:
A function to calculate actual mapgen edges, called from the Mapgen constructor.
A function called indirectly from content_sao.cpp per entity step to check SAO
position is within mapgen edges.

* Calculate borders from params not mapgen, which is not available everytime