Fix math error affecting the linear segment -> polynomial logic (#3562)

Fix typo in the linear segment -> polynomial code

node-graph/libraries/core-types/src/math/polynomial.rs

@@ -203,7 +203,7 @@ impl<const N: usize> Mul for &Polynomial<N> {
 pub fn pathseg_to_parametric_polynomial(segment: PathSeg) -> (Polynomial<4>, Polynomial<4>) {
 	match segment {
 		PathSeg::Line(line) => {
-			let term1 = line.p0 - line.p1;
+			let term1 = line.p1 - line.p0;
 			(Polynomial::new([line.p0.x, term1.x, 0., 0.]), Polynomial::new([line.p0.y, term1.y, 0., 0.]))
 		}
 		PathSeg::Quad(quad_bez) => {

node-graph/libraries/vector-types/src/subpath/lookup.rs

@@ -112,3 +112,17 @@ impl<PointId: Identifier> Subpath<PointId> {
 			.map(|(centroid_part, length)| (DVec2::new(centroid_part.x, centroid_part.y), length))
 	}
 }
+
+#[cfg(test)]
+mod test_centroid {
+	use crate::vector::PointId;
+
+	use super::*;
+	#[test]
+	fn centroid_rect() {
+		let rect = Subpath::<PointId>::new_rect(DVec2::new(100., 100.), DVec2::new(300., 200.));
+		let (centre, area) = rect.area_centroid_and_area(Some(1e-3), Some(1e-3)).unwrap();
+		assert_eq!(area, 200. * 100.);
+		assert_eq!(centre, DVec2::new(200., 150.))
+	}
+}