fix: app crashed when when InWindowBlur switching frequently

[18202781743]

updatePaintNode will be executed multiple times, so it should at least be a slot function that only connects once, and node maybe a invalid pointer, because QSGNode and QuickItem are managed by two different threads. We can use m_item to adjust whether the texture is valid, because it's a QPointer for QQuickItem.

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This pull-request has been approved by: 18202781743, kegechen

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关键摘要:

• 在DBackdropNode的setTextureChangedCallback方法中增加了对m_item的存在性检查，这是一个好的做法，可以避免在回调函数中出现悬空指针的问题。
• 移除了destroyed信号与回调的连接，这可能是为了避免在对象销毁时意外地调用回调函数，但需要确保这一改动不会影响其他依赖于destroyed信号的功能。

是否建议立即修改:

• 是，建议在确认移除destroyed信号连接不会影响系统功能后，进行修改。同时，应该检查是否有必要进行这样的修改，因为通常情况下，我们不会在对象销毁时触发回调函数，除非有特定的理由需要这样做。