FileSystem: Fix file lookup if unrelated files with same name exist

Before, even though we did match all possible extensions,
we only took the first match and tried opening it.
If we were looking for a .png image, but there was an unrelated
.txt file with the same name (as it actually happens in RTPs),
we would potentially see the .txt first, try opening it,
and fail alltogether, even though the image file existed.

Now we try opening all matching files until we find one that
we can parse.

This fixes #101.

src/bitmap.cpp

@@ -233,13 +233,26 @@ struct BitmapPrivate
 	}
 };
 
+struct BitmapOpenHandler : FileSystem::OpenHandler
+{
+	SDL_Surface *surf;
+
+	BitmapOpenHandler()
+	    : surf(0)
+	{}
+
+	bool tryRead(SDL_RWops &ops, const char *ext)
+	{
+		surf = IMG_LoadTyped_RW(&ops, 1, ext);
+		return surf != 0;
+	}
+};
+
 Bitmap::Bitmap(const char *filename)
 {
-	SDL_RWops ops;
-	char ext[8];
-
-	shState->fileSystem().openRead(ops, filename, false, ext, sizeof(ext));
-	SDL_Surface *imgSurf = IMG_LoadTyped_RW(&ops, 1, ext);
+	BitmapOpenHandler handler;
+	shState->fileSystem().openRead(handler, filename);
+	SDL_Surface *imgSurf = handler.surf;
 
 	if (!imgSurf)
 		throw Exception(Exception::SDLError, "Error loading image '%s': %s",