scene/workspacescene: don't assume fullscreen clients are opaque
While the xdg shell specification says otherwise, in practice we have apps that are translucent and expect that to work in fullscreen. More importantly, if we don't enforce opaqueness with GPU compositing, we shouldn't do it in scanout either
(cherry picked from commit 274ef5c9)
Co-authored-by: Xaver Hugl xaver.hugl@gmail.com