Fix last_pos shifting and add unit test

MANIFEST

@@ -58,6 +58,7 @@ t/collinear.t
 t/custom_gcode.t
 t/dynamic.t
 t/fill.t
+t/gcode.t
 t/geometry.t
 t/layers.t
 t/loops.t

lib/Slic3r/GCode.pm

@@ -57,9 +57,10 @@ sub set_shift {
     my $self = shift;
     my @shift = @_;
     
+    # if shift increases (goes towards right), last_pos decreases because it goes towards left
     $self->last_pos->translate(
-        scale ($shift[X] - $self->shift_x),
+        scale ($self->shift_x - $shift[X]),
-        scale ($shift[Y] - $self->shift_y),
+        scale ($self->shift_x - $shift[Y]),
     );
     
     $self->shift_x($shift[X]);

t/gcode.t

@@ -0,0 +1,20 @@
+use Test::More tests => 1;
+use strict;
+use warnings;
+
+BEGIN {
+    use FindBin;
+    use lib "$FindBin::Bin/../lib";
+}
+
+use Slic3r;
+use Slic3r::Geometry qw(scale);
+
+{
+    local $Slic3r::Config = Slic3r::Config->new_from_defaults;
+    my $gcodegen = Slic3r::GCode->new(layer_count => 1);
+    $gcodegen->set_shift(10, 10);
+    is_deeply $gcodegen->last_pos, [scale -10, scale -10], 'last_pos is shifted correctly';
+}
+
+__END__