perf: Do fewer polygon ops

This saves 3.5% of runtime on the sum of all the examples.

surface_vectorial.cpp

@@ -1016,7 +1016,7 @@ vector<multi_polygon_type_fp> Surface_vectorial::offset_polygon(
         buffered_milling_poly = buffered_milling_poly + path_minimum;
       }
     }
-    if (mask) {
+    if (mask && !bg::covered_by(buffered_milling_poly, mask->vectorial_surface->first)) {
       // Don't mill outside the mask because that's a waste.
       // But don't mill into the trace itself.
       // And don't mill into other traces.

testing/gerbv_example/backtrack/expected/test-F.Cu-front-0.3mm.nc

@@ -168,26 +168,34 @@ G01 X5.33697 Y5.53408
 G01 X5.33697 Y4.62587
 G01 X5.38856 Y4.63613
 G01 X5.40578 Y4.58038
+G01 X5.40217 Y4.57944
+G01 X5.40578 Y4.58038
 G01 X5.55061 Y4.18780
 G01 X5.73024 Y3.81726
 G01 X5.72661 Y3.81483
 G01 X5.73024 Y3.81726
 G01 X5.80390 Y3.72610
+G01 X5.80078 Y3.72387
+G01 X5.80390 Y3.72610
 G01 X6.13067 Y3.38582
 G01 X6.35815 Y3.19154
 G01 X6.35569 Y3.18790
 G01 X6.35815 Y3.19154
 G01 X6.51314 Y3.10802
-G01 X6.89737 Y2.94297
+G01 X6.94766 Y2.92311
 G01 X7.17705 Y2.85318
 G01 X7.17626 Y2.84960
 G01 X7.17705 Y2.85318
-G01 X7.41093 Y2.83144
+G01 X7.35620 Y2.83266
 G01 X7.82917 Y2.82994
 G01 X8.06290 Y2.85316
+G01 X8.06369 Y2.84945
+G01 X8.06290 Y2.85316
 G01 X8.29239 Y2.92290
 G01 X8.72678 Y3.10805
 G01 X8.88182 Y3.19154
+G01 X8.88423 Y3.18790
+G01 X8.88182 Y3.19154
 G01 X8.88388 Y3.18854
 G01 X8.88182 Y3.19154
 G01 X9.14814 Y3.42270
@@ -214,31 +222,40 @@ G01 X9.14814 Y6.73727
 G01 X8.88182 Y6.96841
 G01 X8.88423 Y6.97207
 G01 X8.88182 Y6.96841
+G01 X8.88390 Y6.97144
+G01 X8.88182 Y6.96841
 G01 X8.72678 Y7.05193
 G01 X8.34260 Y7.21698
+G01 X8.29231 Y7.23684
 G01 X8.06290 Y7.30679
 G01 X8.06376 Y7.31109
 G01 X8.06290 Y7.30679
+G01 X7.88375 Y7.32696
 G01 X7.82907 Y7.32851
 G01 X7.41093 Y7.32851
+G01 X7.35622 Y7.32696
 G01 X7.17705 Y7.30679
 G01 X7.17619 Y7.31109
 G01 X7.17705 Y7.30679
+G01 X6.94766 Y7.23684
 G01 X6.89737 Y7.21700
 G01 X6.51314 Y7.05193
 G01 X6.35813 Y6.96841
 G01 X6.35569 Y6.97207
 G01 X6.35813 Y6.96841
+G01 X6.13042 Y6.77441
 G01 X6.09183 Y6.73727
 G01 X5.80390 Y6.43387
 G01 X5.73024 Y6.34268
 G01 X5.72658 Y6.34512
 G01 X5.73024 Y6.34268
+G01 X5.57139 Y6.02132
 G01 X5.55061 Y5.97218
 G01 X5.40581 Y5.57959
 G01 X5.38853 Y5.52384
 G01 X5.38416 Y5.52470
 G01 X5.38853 Y5.52384
+G01 X5.36623 Y5.10662
 G01 X5.36666 Y5.05336
 G01 X5.39623 Y5.05645
 G01 X5.43994 Y4.64063