Rhythm Circle - render all notes per instrument

[melodysium]

need a way to render different notes over multiple rotations. as it spins, it fades out old notes and fades in new ones, so it can play through all the notes in a piece

[melodysium]

staged complexity, do one at a time:

1. notes just turn on and off exactly according to their duration
2. notes have some basic ADSR controls for brightness or transparency, but not overlapping with each other
3. notes have ADSR envelopes which can extend outside of their duration
4. notes have little ripples that reach outwards when they are played

planning to just do 1. in this issue, then a follow-up for ADSR and ripples