wayland: Ensure clipboard handling doesn't lock up in certain corner cases
This ensures the clipboard handling doesn't lock up in the following scenarios:
- GTK returned with a mismatching type to the one requested
- GTK fails to convert to the requested type
See also: https://bugzilla.mozilla.org/show_bug.cgi?id=1722369
cc @jadahl per CODEOWNERS
Close #4340 (closed)