Change how children are resize.

Using rect.width & rect.height instead of sizeDelta.
Like that you can have the top container use stretch anchors

--HG--
branch : Ziboo/change-how-children-are-resize-using-rec-1443606808122

Scripts/Layout/HorizontalScrollSnap.cs

@@ -211,7 +211,7 @@ namespace UnityEngine.UI.Extensions
                 RectTransform child = _screensContainer.transform.GetChild(i).gameObject.GetComponent<RectTransform>();
                 currentXPosition = _offset + i * _step;
                 child.anchoredPosition = new Vector2(currentXPosition, 0f);
-                child.sizeDelta = new Vector2(gameObject.GetComponent<RectTransform>().sizeDelta.x, gameObject.GetComponent<RectTransform>().sizeDelta.y);
+                child.sizeDelta = new Vector2(gameObject.GetComponent<RectTransform>().rect.width, gameObject.GetComponent<RectTransform>().rect.height);
             }
 
             _dimension = currentXPosition + _offset * -1;