Fix losing contrast when adjusting 'S' curve of HSL slider
Whe shouldn't change value to fix new saturation position. Human
eye is more precise in lightness, so clamping value will be really
visible to a human eye.
Tha patch also changes the way how 'L' slider is applied. Now it also
does some compensation for 'S' to keep saturation unchanged during the
operation. Otherwise, the image loses some contrast in saturated area.