Provide a way to create a QOpenGLContext from a platform-specific GL context

Currently it's possible to share GL contexts between QGLWidgets. But there's no way to create a QOpenGLContext, QGLContext, or QGLWidget from an existing GL context created outside of Qt.

Say, for example, I have an application that creates its own platform-specific GL context. I'd then like to share this context with a QGLWidget so I can display its output onscreen. There's no way to tell Qt to share with the platform-specific GL context.

It would be great to have a method like QOpenGLContext::createWithPlatformSpecificContext(void *platformSpecificContext). On Mac, I could then cast my CGLContextObj to void * and pass it in to that method, then tell the QGLWidget to share with it.