It is intended that the Pipewire loops are "entered" the whole time they
are running, i.e. are going to process further events. Currently we
enter/leave for single iterations, which is not right.
Fix by entering the loop on initial GSource idle callback, and leaving
at finalize.
That the loop has to be left from the same thread it was entered from is
a bit hard to do with GSources, as the finalize callback appears not
guaranteed to be called from any particular thread. I didn't find a
fully general way to do this, so this puts additional constraints on how
WpLoopSource (and WpCore) are to be cleaned up.
7997fd490b