Unsafe deletion of children QOpenGL* objects from QOpenGLWindow

When using QOpenGLWindow, if one has a QOpenGL wrapper class as a child object of a QOpenGLWindow subclass, then the child object gets destroyed without a context active.

That's merely because ~QObject destroys children first, then the private object (holding the context) is destroyed; and nothing makes the context current in the between. This leads to crashes.