Don't construct overlay screens in ScreenManager ctor. We havn't reset

GAMESTATE by then, so much of it is uninitialized, and we should not load
screens until that's done.  (fixes valgrind warning due to IsEventMode()
being called from SSystemLayer)

stepmania/src/ScreenManager.cpp

@@ -56,13 +56,6 @@ ScreenManager::ScreenManager()
 	m_pSharedBGA = new Actor;
 
 	m_bZeroNextUpdate = false;
-
-	/* By the time this is constructed, THEME has already been set up and set to
-	 * the current theme.  Call ThemeChanged(), to handle the starting theme
-	 * and set up m_SystemLayer. */
-	ASSERT( THEME );
-	ASSERT( !THEME->GetCurThemeName().empty() );
-	this->ThemeChanged();
 }