:: SFX Instruments

    In addition to the 8 built-in instruments, custom instruments can be defined using 
    the first 8 SFX. Use the toggle button to the right of the instruments to select an 
    index, which will show up in the instrument channel as green instead of pink.

    When an SFX instrument note is played, it essentially triggers that SFX, but alters 
    the note's attributes:

        Pitch is added relative to C2
        Volume is multiplied
        Effects are applied on top of the SFX instrument's effects

    For example, a simple tremelo effect could be implemented by defining an instrument 
    in SFX 0 that rapidly alternates between volume 5 and 2. When using this instrument
    to play a note, the volume can further be altered as usual (via the volume channel
    or using the fade in/out effects). In this way, SFX instruments can be used to control 
    combinations of detailed changes in volume, pitch and texture.

    SFX instruments are only retriggered when the pitch changes, or the previous note
    has zero volume. This is useful for instruments that change more slowly over time.
    For example: a bell that gradually fades out. To invert this behaviour, effect 3
    (normally 'drop') can be used when triggering the note. All other effect values have 
    their usual meaning when triggering SFX instruments.