(svn r15685) -Fix [FS#2720]: do not crash when someone substitutes the "map generation" sprites with garbage.

src/heightmap.cpp

@@ -352,7 +352,7 @@ static void GrayscaleToMapHeights(uint img_width, uint img_height, byte *map)
  * This function takes care of the fact that land in OpenTTD can never differ
  * more than 1 in height
  */
-static void FixSlopes()
+void FixSlopes()
 {
 	uint width, height;
 	int row, col;