add ProcessMenuStart for derived screens to trap Start

stepmania/src/ScreenSelectMaster.cpp

@@ -695,6 +695,9 @@ void ScreenSelectMaster::MenuStart( PlayerNumber pn )
 	if( m_bChosen[pn] )
 		return;
 
+	if( !ProcessMenuStart( pn ) )
+		return;
+
 	GameCommand &mc = m_aGameCommands[m_iChoice[pn]];
 
 	/* If no options are playable, then we're just waiting for one to become available.

stepmania/src/ScreenSelectMaster.h

@@ -70,6 +70,7 @@ protected:
 	bool ChangePage( int iNewChoice );
 	bool ChangeSelection( PlayerNumber pn, MenuDir dir, int iNewChoice );
 	float DoMenuStart( PlayerNumber pn );
+	virtual bool ProcessMenuStart( PlayerNumber pn ) { return true; }
 
 	float GetCursorX( PlayerNumber pn, int iPartIndex );
 	float GetCursorY( PlayerNumber pn, int iPartIndex );