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

src/map.cpp

@@ -1378,6 +1378,11 @@ s16 ServerMap::getWaterLevel()
 	return getMapgenParams()->water_level;
 }
 
+bool ServerMap::saoPositionOverLimit(const v3f &p)
+{
+	return getMapgenParams()->saoPosOverLimit(p);
+}
+
 bool ServerMap::blockpos_over_mapgen_limit(v3s16 p)
 {
 	const s16 mapgen_limit_bp = rangelim(
@@ -1838,9 +1843,6 @@ MapBlock *ServerMap::getBlockOrEmerge(v3s16 p3d)
 	return block;
 }
 
-void ServerMap::prepareBlock(MapBlock *block) {
-}
-
 // N.B.  This requires no synchronization, since data will not be modified unless
 // the VoxelManipulator being updated belongs to the same thread.
 void ServerMap::updateVManip(v3s16 pos)