fix polygon perimeter() and restore gyroid filter

src/geo/polygon.js

@@ -1338,7 +1338,7 @@
      * @returns {number} perimeter length (sum of all segment lengths)
      */
     PRO.perimeter = function() {
-        if (this.perim !== 0.0) {
+        if (this.perim) {
             return this.perim;
         }
 

src/mode/fdm/fill.js

@@ -140,7 +140,7 @@
             }
         }
         polys = connectOpenPolys(polys);
-        for (let poly of polys) {
+        for (let poly of polys.filter(p => p.perimeter() > 2)) {
             target.newline();
             for (let point of poly.points) {
                 target.emit(point.x, point.y);