Don't restart song when switching worlds on level select

[NovaSquirrel]

Since the songs are all the same length (64 rows), I think a pretty straightforward solution to this would be to keep track of how many rows the song has gone on for, and then pently_next_row that many times before finally doing the loop in pently_update to write the state to the channels. I think that might be what skip_to_row_top actually does when rehearsal mode is on.