multithreaded realtime audio processing

[atsushieno]

We should be able to process audio in multi-threaded way.

[atsushieno]

It is being implemented on audio-workers branch.

There is an adoption problem on wasm build: there is no multithreaded RT-safe AudioWorklet. There was an issue that brought it in their spec. discussion but it is taken down: WebAudio/web-audio-api#2500

Our wasm backend will keep it disabled.

[atsushieno]

Actually Andriod Oboe is not very different either: we cannot call into plugins' audio process in an RT-safe manner, and Oboe is not multithreaded.

[atsushieno]

It has landed in main tree.