Force altar interaction order

Since #506, it has been possible to interact with an altar *before* it has been solved, in order to hear the melody.

The problem with this is that you can interact with the 2nd altar before solving the 1st, and yet you can't solve the 2nd altar when the 1st is unlit.

I think the sequence should be forced: only when the nth altar has been solved should the (n + 1)th altar be interactive.