Light update for map blocks

This is not really different from the light update of a voxel
manipulator. This update does not assume that the lighting was correct
before, therefore it is useful for correction.

Also expose this function to the Lua API for light correction, and
allow voxel manipulators not to update the light.

src/script/lua_api/l_env.h

@@ -128,6 +128,9 @@ private:
 	// nodenames: eg. {"ignore", "group:tree"} or "default:dirt"
 	static int l_find_nodes_in_area_under_air(lua_State *L);
 
+	// fix_light(p1, p2) -> true/false
+	static int l_fix_light(lua_State *L);
+
 	// emerge_area(p1, p2)
 	static int l_emerge_area(lua_State *L);