This allows the ringbuf to be filled up to `N` instead of just `N-1`, using some fun tricks on the indices.
The advantage is better performance: Before, the first write would fill N-1 bytes, The second would write just the 1 byte left before wrapping, then N-2. Then 2, then N-3, and so on. This would result in more smaller chunks, so worse perf. This problem is gone now.
