fix scroll pointer error

Runtime/gestures/converter.cs

                         Offset scrollDelta = new Offset(_scrollData.scrollX, _scrollData.scrollY) / devicePixelRatio;
                         yield return new PointerScrollEvent(
                             timeStamp: timeStamp,
+                            pointer: state.pointer,
                             kind: kind,
                             device: _scrollData.device,
                             position: position,

Runtime/gestures/events.cs

     public class PointerScrollEvent : PointerSignalEvent {
         public PointerScrollEvent(
             TimeSpan timeStamp,
+            int pointer,
             PointerDeviceKind kind = PointerDeviceKind.mouse,
             int device = 0,
             Offset position = null,
                 kind: kind,
+                pointer: pointer,
                 device: device,
                 position: position) {
             D.assert(position != null);