[proof-of-concept] allow toplevel capture

Missing:
- xwayland child windows
- xwayland unmanaged windows, e.g. popups / menus / ...
- xdg child window positioning
- xdg subsurfaces (test-case: mate-terminal settings listboxes)
- xdg popup positioning

src/foreign-toplevel/ext-foreign.c

@@ -75,6 +75,9 @@ ext_foreign_toplevel_init(struct ext_foreign_toplevel *ext_toplevel,
 		return;
 	}
 
+	/* In support for ext-toplevel-capture */
+	ext_toplevel->handle->data = view;
+
 	/* Client side requests */
 	ext_toplevel->on.handle_destroy.notify = handle_handle_destroy;
 	wl_signal_add(&ext_toplevel->handle->events.destroy, &ext_toplevel->on.handle_destroy);