Add a gradient interpolation attribute with Stepped, Linear (existing), and Smooth modes (#4418)

* Delete dead code function

* Add a Gradient Interpolation axis with Stepped, Linear, and Smooth paths through the stops

* Keep the gradient interpolation attached when dragging stops in the picker

* Give the gradient popover's dropdowns the same tooltips as their row labels

* Order the gradient popover with Intrp. above Space

* Hold gradient stops in place when the cyclic flag is toggled

* Fix wrong Smooth gradient colors around stops sitting on the ramp boundaries

* Consolidate gradient settings plumbing and fix bugs

* Bundle whole-ramp gradient settings into GradientSettings across the editor plumbing

* Name the Smooth gradient interpolation's spline type in its tooltip

* Linearize the gamma hex stop colors recovered from imported Graphite SVGs

* Code review

* Aim the Smooth bake's seam subdivision at the copied boundary color

* Add GradientEvaluator to build gradient sampling state once per loop instead of per sample

* Correct the gradient evaluator's documented setup cost to O(n log n)

node-graph/nodes/graphic/src/graphic.rs

@@ -9,7 +9,7 @@ use rand::SeedableRng;
 use rand::seq::SliceRandom;
 use raster_types::{CPU, GPU, Raster};
 use std::cmp::Ordering;
-use vector_types::gradient::{GradientForm, GradientHueDirection, GradientSpace, GradientSpread};
+use vector_types::gradient::{GradientForm, GradientHueDirection, GradientInterpolation, GradientSpace, GradientSpread};
 use vector_types::{Gradient, ReferencePoint};
 
 /// Returns the list with the item at the specified index removed.
@@ -764,6 +764,23 @@ fn read_attribute_gradient_space(
 	result
 }
 
+/// Reads a named `GradientInterpolation` attribute from the input list, outputting each value as an element of a new `GradientInterpolation[]`.
+#[node_macro::node(category("Attributes: Read"))]
+fn read_attribute_gradient_interpolation(
+	_: impl Ctx,
+	content: ListDyn,
+	/// The attribute name (key) to read.
+	name: Item<String>,
+) -> List<GradientInterpolation> {
+	let name = name.into_element();
+	let mut result = List::with_capacity(content.len());
+	for index in 0..content.len() {
+		let Some(value) = content.attribute::<GradientInterpolation>(&name, index) else { continue };
+		result.push(Item::new_from_element(*value));
+	}
+	result
+}
+
 /// Reads a named `GradientHueDirection` attribute from the input list, outputting each value as an element of a new `GradientHueDirection[]`.
 #[node_macro::node(category("Attributes: Read"))]
 fn read_attribute_gradient_hue_direction(