scene: Add an assert to debug crash in BlurEffect::prePaintWindow()

This checks whether a WindowItem is destroyed in the middle of a painting
cycle.

This is meant as short-term means to debug the issue.

src/scene/workspacescene.cpp

@@ -102,6 +102,14 @@ WorkspaceScene::WorkspaceScene(std::unique_ptr<ItemRenderer> renderer)
         connect(waylandServer()->seat(), &SeatInterface::dragStarted, this, &WorkspaceScene::createDndIconItem);
         connect(waylandServer()->seat(), &SeatInterface::dragEnded, this, &WorkspaceScene::destroyDndIconItem);
     }
+
+    connect(m_containerItem.get(), &Item::childAdded, this, [this](Item *item) {
+        connect(item, &Item::destroyed, this, [this]() {
+            if (m_painting) {
+                qFatal("Destroyed an item while painting");
+            }
+        });
+    });
 }
 
 WorkspaceScene::~WorkspaceScene()
@@ -255,6 +263,7 @@ void WorkspaceScene::frame(SceneDelegate *delegate, OutputFrame *frame)
 
 QRegion WorkspaceScene::prePaint(SceneDelegate *delegate)
 {
+    m_painting = true;
     createStackingOrder();
 
     painted_delegate = delegate;
@@ -384,6 +393,8 @@ void WorkspaceScene::preparePaintSimpleScreen()
 
 void WorkspaceScene::postPaint()
 {
+    m_painting = false;
+
     for (WindowItem *w : std::as_const(stacking_order)) {
         effects->postPaintWindow(w->effectWindow());
     }

src/scene/workspacescene.h

@@ -134,6 +134,7 @@ private:
     std::unique_ptr<Item> m_containerItem;
     std::unique_ptr<Item> m_overlayItem;
     std::unique_ptr<DragAndDropIconItem> m_dndIcon;
+    bool m_painting = false;
 };
 
 } // namespace