Fix `Cxx` to work properly in subpatterns

[SuperDisk]

Q: Can an instrument start at volume 12 fading out at 1 tick per unit, then 6 ticks later retrigger at volume 6 fading out at 2 ticks per unit? A: To retrigger with a new decay rate mid-note, you need to split it into two instruments and track it as two note-ons.