wayland: Fix a crash when switching between aurorae decorations
Aurorae decoration plugin creates an unmapped internal window, it's not rendered and so it's not added to the Workspace.
Internal window setups compositing in its constructor too. It means that the WindowItem will have a link to the scene, but since the Compositor has no any idea about the internal window yet, that scene can become a dangling pointer, and kwin can crash when the window item is destroyed.