compositor will record rendering commands using multiple threads
(i.e., blending commands are recoded using thread pool, and
background one is recorded on aggregate thread).
And there can be temporary refcount increase (so not writable).
Updates fence once all rendering commands have been submitted.
Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/9157>
343cde7f87