global: update generation number also in clients without registry

Not all clients have an existing registry, and the registry generation
number will not be updated for them.  However, we would like to check
for stale globals also elsewhere, eg.  metadata, and it must work also
in this case.

To avoid failing to update client registry generation, on global
addition which the client would see if it had a registry, send done
message for the new global id instead.

src/pipewire/private.h

@@ -300,6 +300,7 @@ struct pw_impl_client {
 	int recv_seq;			/**< last received sequence number */
 	int send_seq;			/**< last sender sequence number */
 	uint64_t recv_generation;	/**< last received registry generation */
+	uint64_t sent_generation;	/**< last sent registry generation */
 
 	void *user_data;		/**< extra user data */