(svn r10143) -Add: store the filename of the grfs opened and allow easy access to the name

-Codechange: store the SpriteID in the spritecache too
-Add: add a PNG loader for graphical files
-Documentation: added a document to explain the PNG format

src/spriteloader/spriteloader.hpp

@@ -26,7 +26,7 @@ public:
 	/**
 	 * Load a sprite from the disk and return a sprite struct which is the same for all loaders.
 	 */
-	virtual bool LoadSprite(SpriteLoader::Sprite *sprite, uint32 file_pos) = 0;
+	virtual bool LoadSprite(SpriteLoader::Sprite *sprite, const char *filename, uint32 file_pos) = 0;
 
 	virtual ~SpriteLoader() { }
 };