Don't use release() on a unique_ptr if we don't intend to release ownership

There is nothing indicating that `OffscreenQuickScene::rootItem()`
releases ownership. Additionally, calling it multiple times currently
means we get a nullptr the second time we call it which crashes the
outputlocator effect.

src/libkwineffects/kwinoffscreenquickview.cpp

@@ -692,7 +692,7 @@ QQmlContext *OffscreenQuickScene::rootContext() const
 
 QQuickItem *OffscreenQuickScene::rootItem() const
 {
-    return d->quickItem.release();
+    return d->quickItem.get();
 }
 
 } // namespace KWin