[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.