Additionally:
- Renamed TIFF_WHITE_LEVEL to DNG_WHITE_LEVEL since it is specified
  in the DNG spec.
- Added/changed some comments to be more precise in differentiating
  between TIFF, TIFF/EP and DNG values.

Related to ticket: https://trac.ffmpeg.org/ticket/4364Signed-off-by: Nick Renieris <velocityra@gmail.com>