Make the colour mixer's saturation reach muted colours

Every photograph with a colour-mixer saturation edit now renders differently:
a raised band is stronger, most of all on muted colours.

The mixer matched bands and judged saturation on scene-linear values, and
scaled chroma by the same factor whatever a colour started at. Against the
photographer's earlier exports of two looks (~90 photographs, their raws, by
encoded hue), a sky band raised by 58 there lifted muted sky blues about
2.1×; here the mixer gave 1.15×, and less in the muted tones that carry most
of a sky or a shadowed snowfield.

Bands are now matched and saturation judged on display-encoded values. A
raised band pushes muted colours hardest and tapers to nothing at full
saturation, at a gain of 3.0, which at the same value lifts muted sky blues
about as those exports did. Lowering saturation still scales every colour
alike. Hue shifts work on the same encoded colour; luminance still scales in
linear light.

The bundled presets that use the mixer, and those whose colour was tuned
against the default rendering, are rescaled to the amount of colour they
had: Vivid 1.30, Vivid warm 1.30, Vivid landscape 1.38, Vivid, strong 1.45,
Vivid portrait 1.15, Punch 1.12, Blue sky 1.08, Deep blue sky 1.12, Polariser
1.26, Blue sky, golden land 1.13 — mean CIELAB chroma over the default
rendering, on 30 raws from the library. Negative values (skin protection)
are left as written.

core/dr-pipeline/presets/essentials.drpl

@@ -25,7 +25,7 @@ vibrance.vibrance = 10
 blacks_whites.blacks = -8
 clarity.amount = 12
 contrast.contrast = 18
-vibrance.vibrance = 32
+vibrance.vibrance = 36
 
 [preset Recover the sky]
 blacks_whites.whites = -10