portal: implement window capture

src/portal/ptl_display.rs

@@ -297,6 +297,7 @@ fn finish_display_connect(dpy: Rc<PortalDisplayPrelude>) {
                     id: dpy.con.id(),
                     con: dpy.con.clone(),
                     owner: Default::default(),
+                    window_capture: Cell::new(false),
                 });
                 dpy.con.add_object(jc.clone());
                 dpy.registry.request_bind(name, version, jc.deref());