Allow setPeriodicWave on Oscillator with "custom" shape

[adamrenklint]

Currently the "custom" shape on Oscillators is pretty useless, since a new Oscillator is created for each note playback in lib/Oscillator#_source and there is no place to hook in to call setPeriodicWave on the actual oscillator node.

So there needs to be some kind of hook there, allowing an Oscillator class instance to define a callback to generate the periodic wave, if the shape is custom.

• https://developer.mozilla.org/en-US/docs/Web/API/OscillatorNode/setPeriodicWave