stream's buffer asynchronously, up to count size.
io.priority can be used to prioritize reads. For the synchronous
version of this function, see gBufferedInputStreamFill.gBufferedInputStreamFillAsync(object, count, io.priority = 0, cancellable = NULL, callback, user.data = NULL)count is -1 then the attempted read size is equal to the number
of bytes that are required to fill the buffer.