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/mapnode.h

@@ -191,6 +191,8 @@ struct MapNode
 		param2 = p;
 	}
 
+	void setLight(enum LightBank bank, u8 a_light, const ContentFeatures &f);
+
 	void setLight(enum LightBank bank, u8 a_light, INodeDefManager *nodemgr);
 
 	/**
@@ -202,6 +204,13 @@ struct MapNode
 
 	u8 getLight(enum LightBank bank, INodeDefManager *nodemgr) const;
 
+	/*!
+	 * Returns the node's light level from param1.
+	 * If the node emits light, it is ignored.
+	 * \param f the ContentFeatures of this node.
+	 */
+	u8 getLightRaw(enum LightBank bank, const ContentFeatures &f) const;
+
 	/**
 	 * This function differs from getLight(enum LightBank bank, INodeDefManager *nodemgr)
 	 * in that the ContentFeatures of the node in question are not retrieved by