Improved lighting

This commit rewrites the procedure that is responsible for light
updating.

this commit
-provides iterative solutions for unlighting and light spreading
-introduces a new priority queue-like container for the iteration
-creates per-node MapBlock caching to reduce retrieving MapBlocks from
the map
-calculates with map block positions and in-block relative node
coordinates
-skips light updating if it is not necessary since the node's new light
will be the same as its old light was

src/map.h

@@ -211,24 +211,10 @@ public:
 			std::set<v3s16> & light_sources,
 			std::map<v3s16, MapBlock*> & modified_blocks);
 
-	void unLightNeighbors(enum LightBank bank,
-			v3s16 pos, u8 lightwas,
-			std::set<v3s16> & light_sources,
-			std::map<v3s16, MapBlock*> & modified_blocks);
-
 	void spreadLight(enum LightBank bank,
 			std::set<v3s16> & from_nodes,
 			std::map<v3s16, MapBlock*> & modified_blocks);
-
-	void lightNeighbors(enum LightBank bank,
-			v3s16 pos,
-			std::map<v3s16, MapBlock*> & modified_blocks);
-
-	v3s16 getBrightestNeighbour(enum LightBank bank, v3s16 p);
-
-	s16 propagateSunlight(v3s16 start,
-			std::map<v3s16, MapBlock*> & modified_blocks);
-
+	
 	void updateLighting(enum LightBank bank,
 			std::map<v3s16, MapBlock*>  & a_blocks,
 			std::map<v3s16, MapBlock*> & modified_blocks);