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.h

@@ -477,6 +477,16 @@ public:
 	u64 getSeed();
 	s16 getWaterLevel();
 
+	/*!
+	 * Fixes lighting in one map block.
+	 * May modify other blocks as well, as light can spread
+	 * out of the specified block.
+	 * Returns false if the block is not generated (so nothing
+	 * changed), true otherwise.
+	 */
+	bool repairBlockLight(v3s16 blockpos,
+		std::map<v3s16, MapBlock *> *modified_blocks);
+
 	MapSettingsManager settings_mgr;
 
 private: