Rework Gradient into a newtype of List<Color> with optional position and midpoint attributes (#4397)

* Rework Gradient into a newtype of List<Color> with optional position and midpoint attributes

* Fix Vello stopless-gradient fallback coverage, empty legacy gradient tables, the node docs gradient swatch, NaN position elision, and wired setter input overwrites

node-graph/nodes/raster/src/adjust.rs

@@ -24,9 +24,7 @@ mod adjust_std {
 	}
 	impl Adjust<Color> for Gradient {
 		fn adjust(&mut self, map_fn: impl Fn(&Color) -> Color) {
-			for color in self.color.iter_mut() {
-				*color = map_fn(color);
-			}
+			*self = self.map_colors(map_fn);
 		}
 	}
 }