Handle audio filtering and output in Javascript

[pdjstone]

Profiling shows that a significant time of each frame is current spent in iir_l and iir_r in sound.c which perform the low-pass audio filtering. I'm not sure why these are so slow, but I suspect it would be more performant to do the filtering using WebAudio, which has an IIR filter built in . Currently audio is done using SDL audio which is translated by Emscripten into WebAudio calls.

Bypassing SDL/Emscripten altogether would hopefully be a performance win, especially if we can use an AudioWorklet to do the work in a separate thread from the main emulation.