view: save view->last_placement on initial positioning

Fix a regression introduced in 3f223fe5 where output layout changes may
move xdg views outside of the output layout with the error:
`view has no last placement info`. This happens also for X11 views if
they don't specify its own geometry on map.

Recent changes introduced `view->last_placement` which is used to
restore view positions after layout changes. This is supposed to be
updated when a view is mapped or manually moved by the user.

However, 3f223fe5 replaced `view_place_by_policy()` with
`view_compute_position_by_policy()` in the xdg view map handler. Unlike
`view_place_by_policy()`, this function does not update
`view->last_placement`, leaving it unset at the time of output layout
changes.

Therefore, this patch adds explicit calls to `view_save_last_placement()`
in the map handler to ensure `view->last_placement` is always set for
mapped views.

Backported from 3bb4ccd22d
Fixes: #3616

src/xdg.c

@@ -128,6 +128,8 @@ do_late_positioning(struct view *view)
 			view->pending.width, view->pending.height,
 			&view->pending.x, &view->pending.y);
 	}
+
+	view_save_last_placement(view);
 }
 
 static void