compositor: add wlr_surface.unmap_commit

This flag can be used to figure out whether a particular commit has
unmapped the surface. Private state for now in case we find a better
way to track this.

include/wlr/types/wlr_compositor.h

@@ -211,6 +211,8 @@ struct wlr_surface {
 		int buffer_width, buffer_height;
 	} previous;
 
+	bool unmap_commit;
+
 	bool opaque;
 	bool has_buffer;
 

types/wlr_compositor.c

@@ -439,7 +439,10 @@ static void surface_commit_state(struct wlr_surface *surface,
 	bool invalid_buffer = next->committed & WLR_SURFACE_STATE_BUFFER;
 
 	if (invalid_buffer && next->buffer == NULL) {
+		surface->unmap_commit = surface->mapped;
 		wlr_surface_unmap(surface);
+	} else {
+		surface->unmap_commit = false;
 	}
 
 	surface_update_damage(&surface->buffer_damage, &surface->current, next);