tablet example: don't update the cursor image on iOS

There is no visible cursor on iOS, so updating the image is a waste
of time.  Removing this does in fact speed up rendering of the strokes.

Change-Id: If958fabeefd4273644707277d4a084855d415bb1
Reviewed-by: Andy Shaw <andy.shaw@qt.io>

examples/widgets/widgets/tablet/tabletcanvas.cpp

@@ -103,8 +103,10 @@ void TabletCanvas::tabletEvent(QTabletEvent *event)
             }
             break;
         case QEvent::TabletMove:
+#ifndef Q_OS_IOS
             if (event->device() == QTabletEvent::RotationStylus)
                 updateCursor(event);
+#endif
             if (m_deviceDown) {
                 updateBrush(event);
                 QPainter painter(&m_pixmap);