Fix refresh problems when scrolling with mousewheel in wxUniv/MSW

Use native scrolling under MSW, just as we already did under X11,
instead of our own version which doesn't work correctly when scrolling
using the mouse wheel.

See https://github.com/wxWidgets/wxWidgets/pull/2525

src/univ/winuniv.cpp

@@ -1061,7 +1061,7 @@ void wxWindow::ScrollWindow(int dx, int dy, const wxRect *rect)
 {
     // use native scrolling when available and do it in generic way
     // otherwise:
-#ifdef __WXX11__
+#if defined(__WXX11__) || defined(__WINDOWS__)
 
     wxWindowNative::ScrollWindow(dx, dy, rect);