Bugfixes and improvements in surface detection

t/clipper.t

@@ -0,0 +1,51 @@
+use Test::More;
+
+plan tests => 1;
+
+use Math::Clipper ':all';
+
+my $clipper = Math::Clipper->new;
+
+my $square = [ # ccw
+    [10, 10],
+    [20, 10],
+    [20, 20],
+    [10, 20],
+];
+
+my $hole_in_square = [  # cw
+    [14, 14],
+    [14, 16],
+    [16, 16],
+    [16, 14],
+];
+
+my $square = [  # ccw
+    [5, 12],
+    [25, 12],
+    [25, 18],
+    [5, 18],
+];
+
+$clipper->add_subject_polygons([ $square, $hole_in_square ]);
+$clipper->add_clip_polygons([ $square ]);
+my $intersection = $clipper->ex_execute(CT_INTERSECTION, PFT_NONZERO, PFT_NONZERO);
+
+is_deeply $intersection, [
+    {
+        holes => [
+            [
+                [14, 16],
+                [16, 16],
+                [16, 14],
+                [14, 14],
+            ],
+        ],
+        outer => [
+            [5, 18],
+            [5, 12],
+            [25, 12],
+            [25, 18],
+        ],
+    },
+], 'hole is preserved after intersection';