improve gap routing. add adaptive shell fallback on failure

src/geo/polygon.js

@@ -1323,15 +1323,15 @@ export class Polygon {
                 pp2.pos = ip2;
             }
         });
-        list.sort(function(p1, p2) {
-            return util.distSq(lp1, p1) - util.distSq(lp1, p2);
-        });
         if (deep && this.inner) {
             this.inner.forEach(p => {
                 let ints = p.intersections(lp1, lp2);
                 if (ints) list.appendAll(ints);
             });
         }
+        list.sort(function(p1, p2) {
+            return util.distSq(lp1, p1) - util.distSq(lp1, p2);
+        });
         return list;
     }
 
@@ -1392,6 +1392,23 @@ export class Polygon {
         return poly;
     }
 
+    /**
+     * emit the shortest poly that connects two points on a closed CW poly
+     * because traveling around the 'end point' may yield a shorter route.
+     * if the poly is open, use emitSegment()
+     */
+    emitShortestSegment(i1, i2) {
+        if (this.open) {
+            return this.emitSegment(i1, i2);
+        }
+        let start = i1.p2.pos,
+            end = i2.p1.pos,
+            points = this.points,
+            seg0 = new Polygon([ i1, ...points.slice(start, end + 1), i2 ]).setOpen(),
+            seg1 = new Polygon([ i2, ...points.slice(end +1 ), ...points.slice(0, start), i1 ]).setOpen();
+        return seg0.perimeter() < seg1.perimeter() ? seg0 : seg1;
+    }
+
     /**
      * @param {Polygon} poly
      * @param {number} [tolerance]