f_output_chain: restore the chain ends manual connections on relink

mp_pin_connect() clears manual_connection on the pins it links, so a
filter that stops being the first or the last one leaves the chain
unable to drive its own ends. Nothing has changed the ends at runtime
so far, so this went unnoticed.

filters/f_output_chain.c

@@ -296,6 +296,9 @@ static void relink_filter_list(struct chain *p)
             mp_pin_connect(f->pins[0], p->filters_out);
         p->filters_out = f->pins[1];
     }
+
+    mp_pin_set_manual_connection_for(p->filters_in, p->f);
+    mp_pin_set_manual_connection_for(p->filters_out, p->f);
 }
 
 static void output_chain_process(struct mp_filter *f)