Fix #6990. Added image collection offsets.

Author: zekeatchan

src/tilemaps/parsers/tiled/BuildTilesetIndex.js

@@ -30,8 +30,12 @@ var BuildTilesetIndex = function (mapData)
         for (var j = 0; j < images.length; j++)
         {
             var image = images[j];
-
-            set = new Tileset(image.image, image.gid, image.width, image.height, 0, 0);
+            var offset = {
+                x: 0,
+                y: image.height - mapData.tileHeight
+            };
+            
+            set = new Tileset(image.image, image.gid, image.width, image.height, 0, 0, null, null, offset);
 
             set.updateTileData(image.width, image.height);