gl/cocoa: Store a weak reference to the `GstGLWindow` instead of the `GstGLContext`

gl/cocoa: Store a weak reference to the `GstGLWindow` instead of the `GstGLContext`

We can't rely on the `GstGLContext` to stay alive and need to keep track
of it. For that we keep track of the `GstGLWindow` in a weak reference
to avoid a reference cycle, and get the corresponding `GstGLContext`
whenever needed.

With contributions from Nirbheek Chauhan.

Fixes https://gitlab.freedesktop.org/gstreamer/gstreamer/-/issues/1697

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3729>