libweston: Add weston_compositor.shutting_down

It does what it says on the box: is true when the compositor is in the
process of shutting down.

Signed-off-by: Daniel Stone <daniels@collabora.com>

include/libweston/libweston.h

@@ -1366,6 +1366,7 @@ struct weston_renderer;
  */
 struct weston_compositor {
 	struct wl_signal destroy_signal;
+	bool shutting_down;
 
 	struct wl_display *wl_display;
 	struct weston_desktop_xwayland *xwayland;

libweston/compositor.c

@@ -8638,6 +8638,8 @@ weston_compositor_shutdown(struct weston_compositor *ec)
 {
 	struct weston_output *output, *next;
 
+	ec->shutting_down = true;
+
 	wl_event_source_remove(ec->idle_source);
 	wl_event_source_remove(ec->repaint_timer);