AOs which use the "push" API must set this field now. Actually, this was
sort of always required, but happened to work anyway. The future
intention is to use device_buffer as the pre-buffer amount, which has to
be available right before audio playback is started. "Pull" AOs really
need this too conceptually, just that the API is underspecified.
From what I can see, only ao_null did not do this yet.