Fix deadlock when exporting is finished, if the project has any Sample TCO(s). (#3742)

include/Mixer.h

@@ -421,6 +421,7 @@ private:
 
 	friend class LmmsCore;
 	friend class MixerWorkerThread;
+	friend class ProjectRenderer;
 
 } ;
 

src/core/ProjectRenderer.cpp

@@ -201,6 +201,9 @@ void ProjectRenderer::run()
 		}
 	}
 
+	// notify mixer of the end of processing
+	Engine::mixer()->stopProcessing();
+
 	Engine::getSong()->stopExport();
 
 	// if the user aborted export-process, the file has to be deleted