Keep the sigmoid as the default curve; Camera Raw's tone is a choice

The Camera Raw default rested on comparing against Lightroom previews
of photographs that carry the user's Lightroom edits — HSL saturation
Blue +58, Aqua +50 and more, Highlights -40, Blacks -20, in every
DNG's XMP — so it measured the house look, not Camera Raw's base
rendering. Under the ACR3 curve _MG_9080 renders brighter than its
Lightroom preview (mean 0.39 against 0.31).

So the curve choice's first variant, the default, is the sigmoid again
and every raw renders as in 0.20.0 apart from baseline exposure. D21
and camera-profiles.md §12 now say the default is open, to be decided
by measuring against Lightroom exports of unedited photographs. Tests
that are about Camera Raw's tone choose it explicitly.

core/dr-gpu/tests/camera_profile.rs

@@ -206,7 +206,12 @@ fn camera_raw_tone_agrees_with_its_cpu_reference() {
         .expect("demosaicer")
         .run(&frame(None))
         .expect("upload");
-    let ops: Vec<Box<dyn Operation>> = vec![Box::new(dr_pipeline::ops::ViewTransform::new())];
+    let mut view = dr_pipeline::ops::ViewTransform::new();
+    view.set_param(
+        dr_pipeline::ops::view_transform::CURVE,
+        dr_pipeline::ops::view_transform::CAMERA_RAW,
+    );
+    let ops: Vec<Box<dyn Operation>> = vec![Box::new(view)];
     let shader = dr_pipeline::compose(&ops);
     let mut adjust = AdjustPass::new(&ctx);
     adjust.render(&source, &shader, SIZE, SIZE).expect("render");