separate song beat for lights, to fix off-sync lights in menus

stepmania/src/GameState.cpp

@@ -668,6 +668,7 @@ void GameState::UpdateSongPosition( float fPositionSeconds, const TimingData &ti
 	ASSERT_M( m_fSongBeat > -2000, ssprintf("%f %f", m_fSongBeat, fPositionSeconds) );
 
 	m_fMusicSeconds = fPositionSeconds;
+	m_fLightSongBeat = timing.GetBeatFromElapsedTime( fPositionSeconds + g_fLightsFalloffSeconds );
 
 	Actor::SetBGMTime( fPositionSeconds, m_fSongBeat );
 	

stepmania/src/GameState.h

@@ -152,6 +152,7 @@ public:
 	float		m_fMusicSeconds;	// time into the current song
 	float		m_fSongBeat;
 	float		m_fCurBPS;
+	float		m_fLightSongBeat; // g_fLightsFalloffSeconds ahead
 	bool		m_bFreeze;	// in the middle of a freeze
 	RageTimer	m_LastBeatUpdate; // time of last m_fSongBeat, etc. update
 	bool		m_bPastHereWeGo;