waylandsink: take into account the case where a pool may be destroyed together with...

waylandsink: take into account the case where a pool may be destroyed together with GstWlDisplay

There are two cases covered here:
1) The GstWlDisplay forces the release of the last buffer and the pool
   gets destroyed in this context, which means it unregisters all the
   other buffers from the GstWlDisplay as well and the display->buffers
   hash table gets corrupted because it is iterating.
2) The pool and its buffers get destroyed concurrently from another
   thread while GstWlDisplay is finalizing and many things get corrupted.