Embedded window doesn't send/receive QPlatformSurfaceEvent in time

When using QWidget::createWindowContainer(QWindow *w) to embed a window in a widget hierarchy and the top level window is closed causing the application to exit, it is found that the QPlatformSurfaceEvent for the embedded window is only created during the QApplication destructor.

However, this is too late as the logic is that events are not delivered during the application destruction procedure.

This means that clients using threaded renderers cannot guard against the platform window surface being destroyed from under them. This can be seen in the Qt 3D basicshapes-cpp example.

We should look to ensure that the QPlatformSurfaceEvent is fired earlier so that event delivery is still in operation and clients can handle it safely.