Use XY.Less comparison func

`XY` has `Less` method to compare two `XY` values. This logic was
duplicated in two other places. This change removes the duplication,
instead relying on the `Less` method.

geom/alg_convex_hull.go

@@ -145,10 +145,7 @@ func convexHullPointSet(g Geometry) []XY {
 
 func monotoneChain(pts []XY) []XY {
 	sort.Slice(pts, func(i, j int) bool {
-		if pts[i].X != pts[j].X {
-			return pts[i].X < pts[j].X
-		}
-		return pts[i].Y < pts[j].Y
+		return pts[i].Less(pts[j])
 	})
 
 	// Calculate lower hull.

geom/line.go

@@ -197,9 +197,7 @@ func rightmostThenHighestIndex(ps []XY) int {
 func leftmostThenLowestIndex(ps []XY) int {
 	rpi := 0
 	for i := 1; i < len(ps); i++ {
-		if ps[i].X < ps[rpi].X ||
-			(ps[i].X == ps[rpi].X &&
-				ps[i].Y < ps[rpi].Y) {
+		if ps[i].Less(ps[rpi]) {
 			rpi = i
 		}
 	}