Fix performance lag caused by scrollbar value set invoke

[whilu]

There is a bug will cause performance lag, the flow like this:

1. When Scroller.UpdatePosition method is called, the custom onValueChanged action been invoked and the scrollbar value will be set.
2. Then the scrollbar onValueChanged action will be invoke, and the Scroller.UpdatePosition method been called again, if the custom onValueChanged action is heavy, there will be some lag for UGUI layout.

Thanks for this lib!