You would also need to make sure the OpenGLContext is made transparent otherwise it will be on top but you still won’t see anything even if you are rendering transparent black.
Check this topic i created years ago:
But in the end i’m afraid that if you really want to start mixing OpenGL and video playback you should consider using FFMPEG or a similar framework to get access to the raw video frames so you can upload them as textures, once you have this you can do whatever you like with the video data and the fun part starts. I know it is not a trivial task but eventually this is the way to go, there is plenty enough resources/projects out there that will help you get started.