Keep model objects aligned to Z = 0 in plater

lib/Slic3r/Print/Object.pm

@@ -332,7 +332,7 @@ sub _slice_region {
     # consider the first one
     $self->model_object->instances->[0]->transform_mesh($mesh, 1);
 
-    # align mesh to Z = 0 and apply XY shift
+    # align mesh to Z = 0 (it should be already aligned actually) and apply XY shift
     $mesh->translate((map unscale(-$_), @{$self->_copies_shift}), -$self->model_object->bounding_box->z_min);
     
     # perform actual slicing

lib/Slic3r/Print/Simple.pm

@@ -68,6 +68,7 @@ sub set_model {
         # if all input objects have defined position(s) apply duplication to the whole model
         $model->duplicate($self->duplicate, $self->_print->config->min_object_distance);
     }
+    $_->translate(0,0,-$_->bounding_box->z_min) for @{$model->objects};
     $model->center_instances_around_point($self->print_center);
     
     foreach my $model_object (@{$model->objects}) {