Fix a crash on startup with QOpenGLWidget

After c4aabeb2b8 we now try to access
q_ptr in QWidgetPrivate::setRenderToTexture. QOpenGLWidget needs to
delay this call after the QObject constructor is done to make sure
that setTextureChildSeen doesn't dereference q_ptr before it has
been initialized.

Change-Id: Icaee82c8b806f42bc7614b0ac6fe4e6026331750
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -60,7 +60,6 @@ public:
     QOpenGLWidgetPrivate()
         : fbo(0), uninitialized(true)
     {
-        setRenderToTexture();
     }
     GLuint textureId() const { return fbo ? fbo->texture() : 0; }
 
@@ -91,6 +90,8 @@ void QOpenGLWidgetPrivate::initialize()
 QOpenGLWidget::QOpenGLWidget(QWidget *parent, Qt::WindowFlags f)
     : QWidget(*(new QOpenGLWidgetPrivate), parent, f)
 {
+    Q_D(QOpenGLWidget);
+    d->setRenderToTexture();
 }
 
 QOpenGLWidget::~QOpenGLWidget()