Many changes and fixes to remove leaks and return objects by reference

xs/t/06_polygon.t

@@ -44,7 +44,7 @@ ok $polygon->is_counter_clockwise, 'is_counter_clockwise';
     ok $clone->is_counter_clockwise, 'make_counter_clockwise';
 }
 
-isa_ok $polygon->first_point, 'Slic3r::Point::Ref', 'first_point';
+ok ref($polygon->first_point) eq 'Slic3r::Point', 'first_point';
 
 # this is not a test: this just demonstrates bad usage, where $polygon->clone gets
 # DESTROY'ed before the derived object ($point), causing bad memory access

xs/t/09_polyline.t

@@ -13,6 +13,7 @@ my $points = [
 ];
 
 my $polyline = Slic3r::Polyline->new(@$points);
+
 is_deeply $polyline->pp, $points, 'polyline roundtrip';
 
 is ref($polyline->arrayref), 'ARRAY', 'polyline arrayref is unblessed';

xs/t/11_clipper.t

@@ -4,7 +4,7 @@ use strict;
 use warnings;
 
 use Slic3r::XS;
-use Test::More tests => 4;
+use Test::More tests => 5;
 
 my $square = [  # ccw
     [200, 100],
@@ -20,6 +20,21 @@ my $hole_in_square = [  # cw
 ];
 my $expolygon = Slic3r::ExPolygon->new($square, $hole_in_square);
 
+{
+    my $result = Slic3r::Geometry::Clipper::offset([ $square, $hole_in_square ], 5);
+    is_deeply [ map $_->pp, @$result ], [ [
+        [205, 95],
+        [205, 205],
+        [95, 205],
+        [95, 95],
+    ], [
+        [155, 145],
+        [145, 145],
+        [145, 155],
+        [155, 155],
+    ] ], 'offset';
+}
+
 {
     my $result = Slic3r::Geometry::Clipper::offset_ex([ @$expolygon ], 5);
     is_deeply $result->[0]->pp, [ [