Prevent FX from producing sound after export

This cuts reverb tails and synth sounds after export.
Disabling freewheeling, continues normal processing where
export left off. This previously kept notes ringing, or reverbs
audible.

libs/ardour/export_handler.cc

@@ -314,8 +314,8 @@ ExportHandler::process_timespan (samplecnt_t samples)
 			export_status->current_postprocessing_cycle = 0;
 		} else {
 			finish_timespan ();
-			return 0;
 		}
+		return 1; /* trigger realtime_stop() */
 	}
 
 	return 0;

libs/ardour/session_export.cc

@@ -239,7 +239,10 @@ Session::process_export (pframes_t nframes)
 	try {
 		/* handle export - XXX what about error handling? */
 
-		ProcessExport (nframes);
+		if (ProcessExport (nframes).value_or (0) > 0) {
+			/* last cycle completed */
+			flush_all_inserts ();
+		}
 
 	} catch (std::exception & e) {
 		error << string_compose (_("Export ended unexpectedly: %1"), e.what()) << endmsg;