fix center calc for tabs in non-circle-like polygons

src/geo/bounds.js

@@ -116,6 +116,10 @@
         return this.maxy - this.miny;
     };
 
+    PRO.center = function(z) {
+        return BASE.newPoint(this.centerx(), this.centery(), z);
+    };
+
     PRO.centerx = function() {
         return this.minx + this.width() / 2;
     };

src/mode/cam/driver.js

@@ -919,7 +919,7 @@
                 let count = proc.camTabsCount,
                     angle = proc.camTabsAngle,
                     angle_inc = 360 / count,
-                    center = trace.circleCenter(),//BASE.newPoint(0,0,slice.z),
+                    center = trace.bounds.center(slice.z),
                     offset = (tabWidth + toolDiam) / 2,
                     ints = [],
                     segs = [];