ANGLE: An openGL window inside widget created with QWidget::createWindowContainer doesn't handle maximizing the parent window properly.

If you have an OpenGL window inside a widget created with QWidget::createWindowContainer and are using ANGLE (i.e. app.setAttribute(Qt::AA_UseOpenGLES)), the OpenGL window doesn't clear out the surface properly when you maximize the parent window. See the attached screenshot. Reproducible with e.g. with ...\Examples\Qt-5.4\widgets\windowcontainer example, if you add the above application attribute setting.