Undocking a dock widget with a QOpenGLWidget turns it into black

Run the attached app. Undock the widget. It becomes empty. Then redock it inside the window: it is rendered correctly again.

Not setting AA_ShareOpenGLContexts results in a crash. The app derives form the 4_0_core version functions and it is not correctly reinitialized when the QOpenGLWidget changes its context.

Setting AA_ShareOpenGLContexts avoids this but then we get empty content.