core: Copy filter clipping setting

Resolves #10997
When filters are duplicated for export, the clip property was
not being copied over. This caused filters that go outside the
layer boundaries (like Drop Shadow and Long Shadow) to be
clipped incorrectly. 
The simple fix was to copy over the clip value when the new
filter was duplicated.

app/core/gimpdrawablefilter.c

@@ -404,6 +404,7 @@ gimp_drawable_filter_duplicate (GimpDrawable       *drawable,
   filter = gimp_drawable_filter_new (drawable, undo_desc, node, icon_name);
   g_object_unref (node);
 
+  gimp_drawable_filter_set_clip (filter, prior_filter->clip);
   gimp_drawable_filter_set_opacity (filter, prior_filter->opacity);
   gimp_drawable_filter_set_mode (filter,
                                  prior_filter->paint_mode,