This changes the semantics of the callbacks so that instead of
a single `requested_frame_count` or `available_frame_count`,
the callbacks get a minimum frame count and maximum frame count.
The callback must write at least the minimum or get an underflow.
The minimum will be 0 on ALSA, PulseAudio, and Dummy, and will
equal the maximum on CoreAudio and JACK.
This ensures optimal behavior regardless of buffer size.