Don’t insert the same layout attributes multiple times

Using pango_attr_list_change is slower, but it prevents spacing attributes to
be set twice, breaking the line-splitting algorithm for some reason.

I don’t know if it is possible to add a simple test for that.

Fix #1025.

weasyprint/text.py

@@ -205,6 +205,8 @@
     void pango_attr_list_unref (PangoAttrList *list);
     void pango_attr_list_insert (
         PangoAttrList *list, PangoAttribute *attr);
+    void pango_attr_list_change (
+        PangoAttrList *list, PangoAttribute *attr);
     PangoAttribute * pango_attr_font_features_new (const gchar *features);
     PangoAttribute * pango_attr_letter_spacing_new (int letter_spacing);
     void pango_attribute_destroy (PangoAttribute *attr);
@@ -776,7 +778,7 @@ def add_attr(start, end, spacing):
                 # TODO: attributes should be freed
                 attr = pango.pango_attr_letter_spacing_new(spacing)
                 attr.start_index, attr.end_index = start, end
-                pango.pango_attr_list_insert(attr_list, attr)
+                pango.pango_attr_list_change(attr_list, attr)
 
             add_attr(0, len(bytestring) + 1, letter_spacing)
             position = bytestring.find(b' ')