Add an API to create QOpenGLContext from an existing native handle

There has to be a way to create a QOpenGLContext with an existing native handle, for example a new static function taking a void*. On platforms that support it, this would result in skipping the actual eglCreateContext and similar calls and instead using the provided handle.

Note that steps like updating the QOpenGLContext's format() must be done also in this case. If dummy windows are needed for this, those must be created with the proper config queried from the provided context.

As for the QPA interface, the cleanest way would most likely be to provide an overload to createPlatformOpenGLContext taking a void*.