Some of the RT parameters that are currently associated to Adjusters are very
hard to edit precisely by dragging the sliders, because small changes to the
default produce quite visible results. Prominent examples include black level,
WB tint, raw black and white points, and lens correction parameters
(distortion, CA, vignetting, perspective). The problem is made worse for those
settings in which not only small changes are significant, but also the
associated Adjusters have a very large range (again, think of black point and
WB tint). This is due to the fact that the current Adjusters have a linear
response. This commit adds an option to use a non-linear (specifically
logarithmic) response, which causes the sliders to move "slowly" around a
designated pivot point, and progressively faster the further you move away
from the pivot.
Besides adding the functionality to the Adjuster class, this changeset also
enables this behaviour for the following adjusters:
- exposure compensation
- black point
- lightness/contrast/saturation/chromaticity (both in exposure and in L*a*b*)
- WB tint
- channel mixer
- lens corrections (perspective, distortion, CA)
- rotation
- raw black and white points
- raw CA correction
