Initialized to `(2 ^ BitsPerSample) - 1` as per the DNG Specification.

Also make sure that `BlackLevel < WhiteLevel`.

This fixes decoding for "X7 CinemaDNG" samples here:
- https://www.dji.com/gr/zenmuse-x7/info#downloadsSigned-off-by: Nick Renieris <velocityra@gmail.com>