:: 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.