Fix stereo -> mono downmixing.

[mutexlox-signal]

Rather than summing the two channels, average them. This prevents integer overflow and clipping or needlessly increasing volume.

This will fix #240.

[mutexlox-signal]

cc @padenot

not sure what the failures are, but they at least seem unrelated? i think?

[mutexlox-signal]

I'm curious whether there's e.g. a breakage with a new version of macOS 14, or a new rustc or something, that is causing the breakage on macOS 14? I looked briefly at the failing tests, and I don't see an obvious connection. I wonder whether running the tests at HEAD on trailblazer would show the same issue.

[padenot]

Historically there's been breakage related to upgrades. https://github.com/mozilla/cubeb-coreaudio-rs/actions/runs/11283677817/job/31383525624 shows that you're right.

[mutexlox-signal]

Thanks!

[mutexlox-signal]

Would you expect that this breakage is scoped just to tests, or do you think it will affect real-world usage as well?

[Pehrsons]

Would you expect that this breakage is scoped just to tests, or do you think it will affect real-world usage as well?

This looks like an issue with the test-only device plugger. Or an issue/difference with macOS 14. Also it's running in a VM and sometimes they seem to get into a weird state because tests that normally don't fail suddenly fail at a high rate.

[mutexlox-signal]

Thanks!