SCI sfx: Let each SongIterator remap its own channels
|Reported by:||SF/lwalkera||Owned by:||bluegr|
Let each SongIterator keep track of its remappings instead of the TeeIterator so that they keep the channel mappings when morphed back to not using a TeeIterator.
Ticket imported from: #2891232. Ticket imported from: patches/1210.