Port offset_ex() and offset2_ex() to XS

xs/t/11_clipper.t

@@ -0,0 +1,53 @@
+#!/usr/bin/perl
+
+use strict;
+use warnings;
+
+use Slic3r::XS;
+use Test::More tests => 2;
+
+my $square = [  # ccw
+    [100, 100],
+    [200, 100],
+    [200, 200],
+    [100, 200],
+];
+my $hole_in_square = [  # cw
+    [140, 140],
+    [140, 160],
+    [160, 160],
+    [160, 140],
+];
+my $expolygon = Slic3r::ExPolygon->new($square, $hole_in_square);
+
+{
+    my $result = @{Slic3r::Geometry::Clipper::offset_ex([ @$expolygon ], 5)};
+    is_deeply $result->[0]->pp, [ [
+        [205, 95],
+        [205, 205],
+        [95, 205],
+        [95, 95],
+    ], [
+        [145, 145],
+        [145, 155],
+        [155, 155],
+        [155, 145],
+    ] ], 'offset_ex';
+}
+
+{
+    my $result = @{Slic3r::Geometry::Clipper::offset2_ex([ @$expolygon ], 5, -2)};
+    is_deeply $result->[0]->pp, [ [
+        [203, 97],
+        [203, 203],
+        [97, 203],
+        [97, 97],
+    ], [
+        [143, 143],
+        [143, 157],
+        [157, 157],
+        [157, 143],
+    ] ], 'offset_ex';
+}
+
+__END__