wayland: Setup compositing for internal window when it's mapped

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.

BUG: 466313
(cherry picked from commit 8fc63468)