This removes the two separate callbacks for sending and receiving data
to/from the device to a unified callback that's used for both input and
output. The new callback takes a pointer to both an input and output
buffer. When the device is opened in playback mode the input pointer
will be set to null. Likewise the output pointer will be set to null
for capture devices. Both input and output pointers will be non-null
for full-duplex devices.
David Reid