Hidden windows with active OpacityAnimators can trigger constrant repaints in other windows

If there are two windows and one of them is hidden and has an active OpacityAnimator that animates the opacity of one of the items in that window, then the other window will constantly update even though it contains no animated items or any other updates.

The attached demo application demonstrates the issue. If you run it as `qml6 test.qml` and then click the "Click" button in the second window, the first window will eventually start updating non stop until the second window is unminimized.

In order to verify that the first window is constantly updated, you could use either the Show Paint effect in KWin or you could run qml6 with WAYLAND_DEBUG=1 environment variable if you're running Wayland or you could just add a qDebug() in QWindow::requestUpdate().

The expected behavior is that the first window will not be repainted constantly after clicking the "Click" button in the second window.