Fix monitor screen cast on X11

Screencast of a monitor broke on X11 at some point, because we started to fetch stage views via MetaRenderer I suspect. This merge requests fixes things up by making the X11 backend create and manage stage views more similarly to the nested and native backend, e.g. created and managed by the MetaRenderer subtype.

Another change is graceful handling of PipeWire crashing during an active screen cast, that I happened to run into by pressing Ctrl+C on pipewire while testing a screen cast.