Don't merge_multi_draws twice.

This was a mistake that wasted a lot of time in processing the gerber
files.

gerberimporter.cpp

@@ -338,16 +338,25 @@ mp_pair merge_multi_draws(const vector<mp_pair>& multi_draws) {
   return merge_multi_draws(new_draws);
 }
 
-multi_polygon_type_fp generate_layers(vector<pair<const gerbv_layer_t *, vector<mp_pair>>>& layers,
+// layers is a vector of layers.  Each layer has a polarity, which can
+// be dark meaning to draw, or clear, meaning to erase.  In the end,
+// the output is regions that are drawn and regions that are undrawn.
+// The layers also have two sets of elements to draw: shapes and
+// filled_closed_regions.  The former are actual shapes.  The later
+// are line drawings that maybe need to be treated as shapes, or not,
+// depend on the options provided.  Finally, there is the xor.  xor
+// overrides the layer polarity if set and causes each layer to be
+// xored with the previous layer, instead of drawn or erased (dark or
+// clear).
+multi_polygon_type_fp generate_layers(vector<pair<const gerbv_layer_t *, mp_pair>>& layers,
                                       multi_polygon_type_fp mp_pair::* member, bool xor_layers) {
   multi_polygon_type_fp output;
   vector<ring_type_fp> rings;
 
   for (auto layer = layers.cbegin(); layer != layers.cend(); layer++) {
     const gerbv_polarity_t polarity = layer->first->polarity;
     const gerbv_step_and_repeat_t& stepAndRepeat = layer->first->stepAndRepeat;
-    const vector<mp_pair>& multi_draws = layer->second;
-    mp_pair draw_pair = merge_multi_draws(multi_draws);
+    mp_pair draw_pair = layer->second;
     multi_polygon_type_fp draws = draw_pair.*member;
 
     // First duplicate in the x direction.
@@ -892,11 +901,16 @@ pair<multi_polygon_type_fp, map<coordinate_type_fp, multi_linestring_type_fp>> G
     }
     linear_circular_paths.clear();
   }
-  auto result = generate_layers(layers, &mp_pair::filled_closed_lines, fill_closed_lines);
+  vector<pair<const gerbv_layer_t *, mp_pair>> merged_layers;
+  merged_layers.reserve(layers.size());
+  for (const auto& layer : layers) {
+    merged_layers.emplace_back(layer.first, merge_multi_draws(layer.second));
+  }
+  auto result = generate_layers(merged_layers, &mp_pair::filled_closed_lines, fill_closed_lines);
   if (fill_closed_lines) {
-    result = result - generate_layers(layers, &mp_pair::shapes, false);
+    result = result - generate_layers(merged_layers, &mp_pair::shapes, false);
   } else {
-    result = result + generate_layers(layers, &mp_pair::shapes, false);
+    result = result + generate_layers(merged_layers, &mp_pair::shapes, false);
   }
 
   if (gerber->netlist->state->unit == GERBV_UNIT_MM) {