Fix a regression introduced by commit 5e79d516 ("ui: split the GL
context in a different object").

Reported-by: Akihiko Odaki <akihiko.odaki@gmail.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Acked-by: Gerd Hoffmann <kraxel@redhat.com>