xwayland: set _NET_WORKAREA property based on usable area

XWayland clients use the _NET_WORKAREA root window property to determine
how much of the screen is not covered by panels/docks. The property is
used for example by Qt to determine areas of the screen that popup menus
should not overlap (see QScreen::availableVirtualGeometry).

Depends on wlroots MR:
https://gitlab.freedesktop.org/wlroots/wlroots/-/merge_requests/4406

v2: prevent calling wlr_xwayland_set_workareas() too early
v3: fix segfault at exit (server->xwayland == NULL)

src/output.c

@@ -24,6 +24,7 @@
 #include "node.h"
 #include "regions.h"
 #include "view.h"
+#include "xwayland.h"
 
 static void
 output_frame_notify(struct wl_listener *listener, void *data)
@@ -610,6 +611,9 @@ output_update_usable_area(struct output *output)
 {
 	if (update_usable_area(output)) {
 		regions_update_geometry(output);
+#if HAVE_XWAYLAND
+		xwayland_update_workarea(output->server);
+#endif
 		desktop_arrange_all_views(output->server);
 	}
 }
@@ -629,6 +633,9 @@ output_update_all_usable_areas(struct server *server, bool layout_changed)
 		}
 	}
 	if (usable_area_changed || layout_changed) {
+#if HAVE_XWAYLAND
+		xwayland_update_workarea(server);
+#endif
 		desktop_arrange_all_views(server);
 	}
 }