Don't mill around the board when cutting traces

node_board/main.cpp

@@ -19,7 +19,8 @@ public:
     {
         png_writer_.allocate(width_px_ + 4 * min_cut_thickness_px_,
                              height_px_ + 4 * min_cut_thickness_px_);
-        png_writer_.set_all_pixels_black();
+        png_writer_.set_all_pixels(255);
+        draw_int_rectangle(0, width_px_, 0, height_px_, 0);
     }
 
     int32_t to_px(double x) {