Add a hash mechanism to BaseMapSource.

This is needed to recognize what type of tiles a layer contains.
Client code has to fill in a map of hash => add_layer<T>() pointers.
As a layer is loaded, the correct add_layer<T>() is called based
on the hash saved with the layer itself.

include/doorkeeper/helpers/asciimapsource.hpp

@@ -33,6 +33,7 @@ namespace dkh {
 		virtual int layersCount ( void ) const;
 		virtual const coords& tileSize ( void ) const;
 		virtual void chainedMaps ( std::vector<std::string>& parOut ) const;
+		virtual dk::HashType layerTypeHash ( int parIndex ) const;
 
 	private:
 		void parse_map_data ( std::istream& parSrc );