Ship four presets for a bluer sky

Blue sky, Deep blue sky, Polariser, and Blue sky with golden land, in a
Skies section after Essentials. Each darkens the colour mixer's azure and
blue bands and adds chroma to them — what a polarising filter does to a
clear sky — and brings the highlights down with it, so a white cloud does
not read as a cut-out against the deeper blue. The stronger ones nudge
azure towards blue and add dehaze.

They are looks and work only on the hues a sky occupies, so an overcast
frame is left nearly alone: there is no blue for them to deepen, and
tinting grey cloud blue would be worse than doing nothing. Tuned by eye on
the demo library's alpine and Manhattan frames, with an overcast Étretat
frame as the control.

core/dr-pipeline/presets/skies.drpl

@@ -0,0 +1,52 @@
+drpl 1
+
+# Skies: a bluer, deeper sky without touching the rest of the picture.
+#
+# Written against this pipeline, not derived from anybody's preset. Each
+# works the colour mixer's azure and blue bands — the hues a clear sky
+# occupies, 210° and 240° — darkening them and adding chroma, which is what
+# a polarising filter does to a sky and why it reads as "more blue" rather
+# than "more saturated". A grey sky has no hue for the bands to find, so on
+# an overcast frame these do little, by construction: they cannot invent a
+# sky, and a preset that tinted grey clouds blue would be one nobody trusted.
+#
+# Highlights come down with the sky in the stronger ones, because a darker
+# blue beside a clipped white cloud looks like a mask edge.
+
+[preset Blue sky]
+colour_mixer.azure_lum = -20
+colour_mixer.azure_sat = 25
+colour_mixer.blue_lum = -15
+colour_mixer.blue_sat = 20
+highlights_shadows.highlights = -15
+
+[preset Deep blue sky]
+colour_mixer.azure_hue = 10
+colour_mixer.azure_lum = -30
+colour_mixer.azure_sat = 35
+colour_mixer.blue_lum = -25
+colour_mixer.blue_sat = 30
+colour_mixer.cyan_sat = 10
+highlights_shadows.highlights = -30
+
+[preset Polariser]
+colour_mixer.azure_hue = 10
+colour_mixer.azure_lum = -35
+colour_mixer.azure_sat = 40
+colour_mixer.blue_lum = -30
+colour_mixer.blue_sat = 35
+colour_mixer.cyan_lum = -10
+colour_mixer.cyan_sat = 15
+dehaze.amount = 20
+highlights_shadows.highlights = -35
+vibrance.vibrance = 10
+
+[preset Blue sky, golden land]
+colour_mixer.azure_lum = -20
+colour_mixer.azure_sat = 25
+colour_mixer.blue_lum = -15
+colour_mixer.blue_sat = 20
+colour_mixer.orange_sat = 12
+colour_mixer.yellow_hue = -10
+colour_mixer.yellow_sat = 15
+highlights_shadows.highlights = -20

core/dr-pipeline/src/bundled.rs

@@ -62,6 +62,7 @@ const SECTIONS: &[(&str, &str, &str)] = &[
         "Essentials",
         include_str!("../presets/essentials.drpl"),
     ),
+    ("skies", "Skies", include_str!("../presets/skies.drpl")),
     (
         "colour_film",
         "Colour film",