[NUI] Fix ScrollTo to move pan position to the argument position (#1652)

[NUI] Fix ScrollTo to move pan position to the argument position (#1652)

Previously, ScrollTo moved pan position including the ScrollableBase's
current position.
e.g. ScrollTo(360, true); //moves by 360
     ScrollTo(0, true); //moves by 360 again based on current position

The argument position is the new pan position. So the new position of
ScrollableBase becomes (-position).
To move ScrollableBase's position to (-position), it moves by
(-position - currentPosition).

Co-authored-by: Jaehyun Cho <jae_hyun.cho@samsung.com>