start adding threading to SLA

src/geo/polygons.js

@@ -29,6 +29,7 @@
         trimTo,
         expand,
         expand_lines,
+        points,
         route,
         union,
         inset,
@@ -145,6 +146,10 @@
         return output;
     }
 
+    function points(polys) {
+        return polys.length ? polys.map(p => p.deepLength).reduce((a,v) => a+v) : 0;
+    }
+
     /**
      * todo use clipper polytree?
      *