scene: Sync Window::readyForPainting() to window item visibility

If a window is made ready for painting with some delay, the item won't
schedule an update. To fix that, sync the item visibility state with
Window::readyForPainting(). When the item visibility changes, a repaint
will be scheduled.

BUG: 464955

src/scene/windowitem.cpp

@@ -126,6 +126,9 @@ void WindowItem::handleWindowClosed(Window *original, Deleted *deleted)
 
 bool WindowItem::computeVisibility() const
 {
+    if (!m_window->readyForPainting()) {
+        return false;
+    }
     if (waylandServer() && waylandServer()->isScreenLocked()) {
         return m_window->isLockScreen() || m_window->isInputMethod() || m_window->isLockScreenOverlay();
     }

src/scene/workspacescene.cpp

@@ -443,13 +443,9 @@ void WorkspaceScene::createStackingOrder()
     // window should not get focus before it's displayed, handle unredirected windows properly and
     // so on.
     for (Window *window : std::as_const(windows)) {
-        if (!window->readyForPainting()) {
-            continue;
+        if (window->windowItem()->isVisible()) {
+            stacking_order.append(window->windowItem());
         }
-        if (!window->windowItem()->isVisible()) {
-            continue;
-        }
-        stacking_order.append(window->windowItem());
     }
 }