WebAudio: Fix a noise sound before device was started

[FrancoisRecisio]

Hello! In our project compiled with Emscripten and executed in a browser, we had a noise sound at startup. We found that it occurred after the device was init and before it was started. And if we don't start the device, we can hear the never-ending noise. This code fixes that: if the device is not started, the output buffer is filled with 0.

[FrancoisRecisio]

I updated this PR, we still had the problem (but I don't know if there was a version regression on our side...). So now I use a loop to iterate through outputs, rather than having a single MA_ZERO_MEMORY. Or maybe pDevice->playback.internalChannels = pOutputs[i].numberOfChannels * outputCount ?

On another topic, emscripten has exposed samplesPerChannel instead of 128, since version 3.1.70. This could be the subject of another PR but it would be great to use.

[mackron]

Thanks for this. This looks fine to me on the surface. I've just switched from Windows to Linux and haven't yet moved all of my projects over to my new workflow so might be a bit before I get to this.

On another topic, emscripten has exposed samplesPerChannel instead of 128, since version 3.1.70. This could be the subject of another PR but it would be great to use.

Nice! Thanks a lot for the tip on that. I actually mentioned this to the Emscripten developers a while ago, so I'll definitely be open to integrating that (my issue if you care: emscripten-core/emscripten#18853).