iOS: Call setNeedsLayout() in scrollViewDidScroll()

Otherwise, the web view scrolls in blank white space which
doesn't get painted until the scroll gesture is finished,
because the WKWebView hasn't rendered that part of the
viewport to its offscreen buffer.

ios/Classes/InAppWebView.swift

@@ -622,6 +622,7 @@ public class InAppWebView: WKWebView, UIScrollViewDelegate, WKUIDelegate, WKNavi
             let y = Int(scrollView.contentOffset.y / scrollView.contentScaleFactor)
             onScrollChanged(x: x, y: y)
         }
+        setNeedsLayout()
     }
     
     public func onLoadStart(url: String) {