Shaders: graster-nodes no-std prep (#2924)

* raster-nodes: remove commented out index node

* raster-nodes: move `CubicSplines` to separate mod

* raster-nodes: create `mod blending_nodes` and move assoc nodes

* raster-nodes: move node `gradient_map` to its own mod

node-graph/graster-nodes/src/gradient_map.rs

@@ -0,0 +1,30 @@
+//! Not immediately shader compatible due to needing [`GradientStops`] as a param, which needs [`Vec`]
+
+use crate::adjust::Adjust;
+use graphene_core::gradient::GradientStops;
+use graphene_core::raster_types::{CPU, RasterDataTable};
+use graphene_core::{Color, Ctx};
+
+// Aims for interoperable compatibility with:
+// https://www.adobe.com/devnet-apps/photoshop/fileformatashtml/#:~:text=%27grdm%27%20%3D%20Gradient%20Map
+// https://www.adobe.com/devnet-apps/photoshop/fileformatashtml/#:~:text=Gradient%20settings%20(Photoshop%206.0)
+#[node_macro::node(category("Raster: Adjustment"))]
+async fn gradient_map<T: Adjust<Color>>(
+	_: impl Ctx,
+	#[implementations(
+		Color,
+		RasterDataTable<CPU>,
+		GradientStops,
+	)]
+	mut image: T,
+	gradient: GradientStops,
+	reverse: bool,
+) -> T {
+	image.adjust(|color| {
+		let intensity = color.luminance_srgb();
+		let intensity = if reverse { 1. - intensity } else { intensity };
+		gradient.evaluate(intensity as f64).to_linear_srgb()
+	});
+
+	image
+}