TileGenerator: free database resources (#38)

Destructor of DB* instance was never called.
Ensure it is, adding missing base class virtual destructor and calling delete when possible to free resources.

TileGenerator.h

@@ -76,6 +76,7 @@ public:
 private:
 	void parseColorsStream(std::istream &in);
 	void openDb(const std::string &input);
+	void closeDatabase();
 	void loadBlocks();
 	void createImage();
 	void renderMap();