Workaround dclone() not being thread-safe

lib/Slic3r/ExPolygon.pm

@@ -32,6 +32,11 @@ sub clone {
     Storable::dclone($_[0])
 }
 
+sub threadsafe_clone {
+    my $self = shift;
+    return (ref $self)->new(map $_->threadsafe_clone, @$self);
+}
+
 sub contour {
     my $self = shift;
     return $self->[0];
@@ -303,7 +308,7 @@ has 'expolygons' => (is => 'ro', default => sub { [] });
 sub clone {
     my $self = shift;
     return (ref $self)->new(
-        expolygons => [ map $_->clone, @{$self->expolygons} ],
+        expolygons => [ map $_->threadsafe_clone, @{$self->expolygons} ],
     );
 }