fix: Visualize some sliders better:

- Balance sliders dont get highlighted at the left side of the handle, does not make sense for them
- Slider highlighting is done by luminosity change only, so colored sliders retain their color (use CAIRO_OPERATOR_HSL_LUMINOSITY)
- split toning's balance is now also colored in the two colors to balance between
- increase slider coloring slightly from 0.2f to 0.25f