backend/sys: Use `WL_MARSHAL_FLAG_DESTROY` where appropriate

[ids1024]

Presumably the race that can happen in https://gitlab.freedesktop.org/wayland/wayland/-/commit/0e0274af0c9f60d2759713df136f4294054c9096 can happen in wayland-rs (at least when racing against non-rust code sharing a connection), so we should presumably do the same thing.