Make QGLWidget::paintGL() call glClear().

Creating and displaying a plain QGLWidget on Mac would
display "garbage" or previous frame buffer content
on screen. This looks broken and raises interesting 
privacy concerns.

Fix by adding a call to glClear().

Change-Id: I507c24275e41fac0be5f518c5a70d151099ae6b8
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/opengl/qgl.cpp

@@ -3854,6 +3854,7 @@ void QGLWidget::initializeGL()
 
 void QGLWidget::paintGL()
 {
+    glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
 }