Sometimes these callbacks will fire while we're still waiting on state to
settle down in PIPEWIRE_OpenDevice, which means we're holding the device lock,
but then the i/o callback will fire from a background thread and also try to
grab the device lock, but can't, because PIPEWIRE_OpenDevice is holding it and
waiting for this i/o callback to finish...hence, a deadlock.
So now, if the device is still opening, output callbacks will write silence
and input callbacks will just flush the buffer, without calling the main
iterate function, and thus avoid obtaining the lock.
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