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).

[mackron]

I've finally got around to merging this. I also added support for the samplesPerChannel thing as well. Older versions of Emscripten are still supported. This was the change in case you were interested: ae2cd4b

The way I understand it is that Emscripten currently supports the ability to query the quantum size, but you can't actually configure it just yet. The internal infrastructure to support that is now in place though so it should be trivial to plug that in once Emscripten adds support.

[FrancoisRecisio]

Thanks for the merge and the variable buffer sizes!