winrt: Work around broken screen rotation on Windows Phone 8.1.

Expose on orientation change clears bad frames caused by race between
Qt and ANGLE.

Change-Id: I8970c6be36133d861c718a946d9e9b7a5e88cc2c
Task-Id: QTBUG-44333
Reviewed-by: Andrew Knight <andrew.knight@intopalo.com>

src/plugins/platforms/winrt/qwinrtscreen.cpp

@@ -1048,6 +1048,7 @@ HRESULT QWinRTScreen::onOrientationChanged(IDisplayInformation *, IInspectable *
     if (d->orientation != newOrientation) {
         d->orientation = newOrientation;
         QWindowSystemInterface::handleScreenOrientationChange(screen(), d->orientation);
+        handleExpose(); // Clean broken frames caused by race between Qt and ANGLE
     }
     return S_OK;
 }