waylandsink: Handle wl_buffer::release and don't reuse buffers that are not released

waylandsink: Handle wl_buffer::release and don't reuse buffers that are not released

This is achieved by adding an extra reference on the buffers, which does
not allow them to return to the pool. When they are released, this reference
is dropped.

The rest complexity of this patch (hash table, mutex, flag, explicit release calls)
merely exists to allow a safe, guaranteed and deadlock-free destruction sequence.
See the added comment on gstwaylandsink.c for details.