The `videosink` refernce in main() is a floating one, so it should not
be unref'ed (the playbin practically takes ownership of it).
This prevents a "gst_object_unref: assertion '((GObject *)
object)->ref_count > 0' failed" at runtime.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/6883>
On macOS, a Cocoa event loop is needed in the main thread to ensure
things like opening a GL window work correctly. In the past, this was
patched into glib via Cerbero, but that prevented us from updating it.
This workaround simply runs an NSApplication and then calls the
main function on a secondary thread, allowing GStreamer to correctly
display windows and/or system permission prompts, for example.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/3532>
