[scene] Setup scene window connections with correct receiver object
We need a couple of connections to ensure that the window pixmap, the window quad cache, and the window shape get discarded when the geometry of the toplevel has been changed. Currently, those connections are created with the receiver object being the scene. The problem is that the associated wayland surface may outlive the toplevel and we don't cleanup the connections after the scene window has been destroyed. The fact that the connections don't get destroyed can lead to accessing dangling pointers, which may result in a crash. In order to ensure that the connections are broken automatically when the scene window is destroyed, we need to ensure that the received object is the scene window. That way, the connections will be destroyed automatically.
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