toplevel-capture: partial initial implementation

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/xdg-popup.c

@@ -169,4 +169,6 @@ xdg_popup_create(struct view *view, struct wlr_xdg_popup *wlr_popup)
 
 	node_descriptor_create(wlr_popup->base->surface->data,
 		LAB_NODE_XDG_POPUP, view, /*data*/ NULL);
+
+	wlr_scene_xdg_surface_create(&view->capture.scene->tree, wlr_popup->base);
 }