Fix reverse indexed iterations

gfx/prop/children.lua

@@ -13,11 +13,11 @@ function Children:with(elementType)
   function elementType:_iterateChildrenReversed()
     local children = self:_children()
     return function(tbl, idx)
-      if idx < 1 then
+      if idx <= 1 then
         return nil, nil
       end
-      return idx - 1, tbl[idx]
-    end, children, #children
+      return idx - 1, tbl[idx - 1]
+    end, children, #children + 1
   end
 
   function elementType:_children()