Like AudioBufferSourceNode and ScriptProcessorNode, OscillatorNode
holds a self-reference in mPlaying. Keep track of all osc nodes
in the AudioContext and explicitly stop them to make sure this
reference is dropped.
Simple direct calculation of the fixed types.
We probably want to use the PeriodicWave machinery to generate
the pre-defined waveforms once it's available. In either case
we can optimize this significantly by caching the generated
waveform when the frequency is stationary over the block.