const-correctness for Song::getPlayPos()

include/Song.h

@@ -186,6 +186,10 @@ public:
 	{
 		return m_playPos[pm];
 	}
+	inline const PlayPos & getPlayPos( PlayModes pm ) const
+	{
+		return m_playPos[pm];
+	}
 
 	void updateLength();
 	tact_t length() const

src/core/ProjectRenderer.cpp

@@ -163,7 +163,7 @@ void ProjectRenderer::run()
     //skip first empty buffer
     Engine::mixer()->nextBuffer();
 
-	Song::PlayPos & pp = Engine::getSong()->getPlayPos(
+	const Song::PlayPos & pp = Engine::getSong()->getPlayPos(
 							Song::Mode_PlaySong );
 	m_progress = 0;
 	const int sl = ( Engine::getSong()->length() + 1 ) * 192;