fix broken GradeDisplay

stepmania/src/Grade.cpp

@@ -8,6 +8,19 @@
 
 LuaXType( Grade );
 
+ThemeMetric<int> NUM_GRADE_TIERS_USED("PlayerStageStats","NumGradeTiersUsed");
+
+Grade GetNextPossibleGrade( Grade g )
+{
+	if( g < NUM_GRADE_TIERS_USED - 1 )
+		return (Grade)(g+1);
+	else if( g == NUM_GRADE_TIERS_USED - 1 )
+		return Grade_Failed;
+	else
+		return Grade_Invalid;
+}
+
+
 RString GradeToLocalizedString( Grade g )
 {
 	RString s = GradeToString(g);