xdg_shell stable: allow zero-sized positioner set_anchor_rect

This is a protocol difference with xdg-shell-unstable-v6

types/wlr_xdg_shell.c

@@ -252,10 +252,10 @@ static void xdg_positioner_protocol_set_anchor_rect(struct wl_client *client,
 	struct wlr_xdg_positioner *positioner =
 		wl_resource_get_user_data(resource);
 
-	if (width < 1 || height < 1) {
+	if (width < 0 || height < 0) {
 		wl_resource_post_error(resource,
 			XDG_POSITIONER_ERROR_INVALID_INPUT,
-			"width and height must be positives and non-zero");
+			"width and height must be positives");
 		return;
 	}