add top switching on next layer when previous poly was short

src/kiri/print.js

@@ -21,6 +21,7 @@ class Print {
         this.settings = settings;
         this.widgets = widgets;
         this.lastPoint = null;
+        this.lastPoly = null;
         this.lastEmit = null;
         this.lastOut = null;
         this.lastPos = null;
@@ -116,6 +117,8 @@ class Print {
             last = point;
         }, close, closest.index);
 
+        this.lastPoly = poly;
+
         return output.last().point;
     }
 
@@ -548,6 +551,11 @@ class Output {
         }
         return o;
     }
+
+    set_retract() {
+        this.retract = true;
+        return this;
+    }
 }
 
 function newPrint(settings, widgets, id) {