Don’t flicker on startup: render on expose

All QWindow implementations must produce a frame before
returning from the expose event.

Change-Id: I5640809c47e948101879c1623c12230fa0a63c44
Reviewed-by: Tor Arne Vestbø <tor.arne.vestbo@qt.io>

examples/opengl/hellowindow/hellowindow.cpp

@@ -93,6 +93,8 @@ HelloWindow::HelloWindow(const QSharedPointer<Renderer> &renderer, QScreen *scre
 void HelloWindow::exposeEvent(QExposeEvent *)
 {
     m_renderer->setAnimating(this, isExposed());
+    if (isExposed())
+        m_renderer->render();
 }
 
 void HelloWindow::mousePressEvent(QMouseEvent *)

examples/opengl/hellowindow/hellowindow.h

@@ -70,7 +70,7 @@ public:
 
     void setAnimating(HelloWindow *window, bool animating);
 
-private slots:
+public slots:
     void render();
 
 private: