Avoid drawing when the QOpenGLWidget isn't mapped

Do as QGLWidget::updateGL does and avoid calling paintGL in updateGL
when we we don't have a valid QSurface to draw on.

We currently end up calling makeCurrent on the context with a null
QSurface in that case, which is the equivalend of doneCurrent, and
causes QOpenGLContext::currentContext to return null later when paintGL
is called on the subclass.

Change-Id: I712ee59274855457b0e4de3757754f56b3528955
Reviewed-by: Laszlo Agocs <laszlo.agocs@digia.com>
Reviewed-by: Paul Olav Tvete <paul.tvete@digia.com>

src/widgets/kernel/qopenglwidget.cpp

@@ -147,7 +147,7 @@ void QOpenGLWidget::paintGL()
 void QOpenGLWidget::updateGL()
 {
     Q_D(QOpenGLWidget);
-    if (d->uninitialized)
+    if (d->uninitialized || !d->surface())
         return;
 
     makeCurrent();