digikam from trunk : Improvement of Brighness/Contrast/gamma image editor tool :

- Simplification of BCG filter algorithm.
- Speed-up computation of filter.
- Fix Over-Exposure indicator prediction.
- Fix BCG matrix computation with negative values (it's a side-effect bug with old algorithm introduced by Renchi Raju)
- Add a new Under-Exposure indicator!
- All settings value excursions are now the same than Photoshop.