add new path generator. let it bake for a while before removing the old one

src/geo/polygon.js

@@ -3,6 +3,7 @@
 "use strict";
 
 // dep: geo.base
+// dep: geo.paths
 // dep: ext.clip2
 // dep: ext.earcut
 // dep: geo.point
@@ -59,6 +60,14 @@ class Polygon {
         return bounds;
     }
 
+    toPath2D(offset) {
+        return base.paths.pointsToPath(this.points, offset, this.open);
+    }
+
+    toPath3D(offset, height, z) {
+        return base.paths.pathTo3D(this.toPath2D(offset), height, z);
+    }
+
     toString(verbose) {
         let l;
         if (this.inner && this.inner.length) {