Only create autogen steps pointers if steps autogen is enabled. This fixes

random courses taking too long to generate with autogen disabled: it has
to iterate over *all* steps (including the unused autogen steps); when
outputting the catalog, this means it has to iterate over every song once
per random course line per steps type per course difficulty.  (test case
of one 9-random-entry course with autogen off: ~10 seconds -> ~1.5 seconds;
still too much)

stepmania/src/Song.cpp

@@ -1091,6 +1091,10 @@ void Song::AddAutoGenNotes()
 		if( HasNotes[stMissing] )
 			continue;
 
+		/* If m_bAutogenSteps is disabled, only autogen lights. */
+		if( !PREFSMAN->m_bAutogenSteps && stMissing != STEPS_TYPE_LIGHTS_CABINET )
+			continue;
+
 		// missing Steps of this type
 		int iNumTracksOfMissing = GAMEMAN->NotesTypeToNumTracks(stMissing);