[FEATURE] Implemented a composite RGB curve for Curves filter
Now the curves filter provides two ways of adjusting image brightness:
- Adjusting a grouped RGB curve
- Adjusting L channel of Lab separation of the image using 'Lightness' curve
http://nonaynever.ru/kwiki/krita/published/L_vs_RGB_curve_comparison
Ref T352
CC:kimageshop@kde.org