Improved calculation of first line for song name text

With issue pull request #455 I just implemented a simple calculation based on the song gap. But on some txt files the first line don't start at beat 0. Improved the calculation to the first beat of the first line from voice 0.

Vocaluxe/Screens/CScreenSing.cs

@@ -1187,7 +1187,11 @@ namespace Vocaluxe.Screens
             _TimerDuetText1.Reset();
             _TimerDuetText2.Reset();
 
-            float realGap = song.Gap - song.Start;
+            // Getting the real start of the first line from the first voice, no other voices are on top since #454.
+            // Needed for displaying song name on duet game mode or lyric on top settings (implemented with #455).
+            CVoice voice = song.Notes.GetVoice(0);      // As long as only voice 0 is on top (no matter if duet or other modes).
+            CSongLine[] lines = voice.Lines;
+            float realGap = CGame.GetTimeFromBeats(lines[0].FirstNoteBeat, song.BPM) + (song.Gap - song.Start);
             _TimeShowSongTextTotal = 10f;
             if (_Lyrics[_LyricMainDuet].Visible || _Lyrics[_LyricMainTop].Visible)
             {