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/map.cpp

@@ -2591,6 +2591,16 @@ void ServerMap::PrintInfo(std::ostream &out)
 	out<<"ServerMap: ";
 }
 
+bool ServerMap::repairBlockLight(v3s16 blockpos,
+	std::map<v3s16, MapBlock *> *modified_blocks)
+{
+	MapBlock *block = emergeBlock(blockpos, false);
+	if (!block || !block->isGenerated())
+		return false;
+	voxalgo::repair_block_light(this, block, modified_blocks);
+	return true;
+}
+
 MMVManip::MMVManip(Map *map):
 		VoxelManipulator(),
 		m_is_dirty(false),