Implement SDL audio buffer queueing to use Web Audio ScriptProcessorNode.

Implement SDL audio buffer queueing to use Web Audio ScriptProcessorNode in the special case when the requested SDL_OpenAudio sampling rate matches the Web Audio graph native sampling rate. Using ScriptProcessorNode should have the benefit that it pulls audio buffer updates with better granularity/precision than using setTimeout, which suffers from too much jitter (even spikes as long as 230msecs seen).

This is tested by the existing browser.test_sdl_audio_beeps, which passes.