ProjectRenderer: start thread with normal priority

There's no need to start the ProjectRenderer thread with high priority
anymore as the actual rendering is done on the other side of the FIFO.
The ProjectRenderer just waits for new data in the FIFO and encodes
them as they arrive.

src/core/ProjectRenderer.cpp

@@ -22,7 +22,6 @@
  *
  */
 
-
 #include <QtCore/QFile>
 #include <QTimer>
 
@@ -157,11 +156,7 @@ void ProjectRenderer::startProcessing()
 		// called immediately
 		engine::mixer()->setAudioOutputContext( m_context );
 
-		start(
-#ifndef LMMS_BUILD_WIN32
-			QThread::HighPriority
-#endif
-						);
+		start();
 	}
 }