Fix rasterwindow example

At least with the eglfs platform plugin, the QBackingStore constructor
results in a null pointer access if done before creation.

Change-Id: I2e78e70700fa48499a35c55797e1b962b6e6285a
Reviewed-by: Rebecca Worledge <rebecca.worledge@theqtcompany.com>
Reviewed-by: Andy Nichols <andy.nichols@qt.io>

examples/gui/rasterwindow/rasterwindow.cpp

@@ -45,8 +45,8 @@ RasterWindow::RasterWindow(QWindow *parent)
     : QWindow(parent)
     , m_update_pending(false)
 {
-    m_backingStore = new QBackingStore(this);
     create();
+    m_backingStore = new QBackingStore(this);
 
     setGeometry(100, 100, 300, 200);