gst_tag_list_add and gst_tag_list_add_values (and their
*_valist variants) behave inconsistently when the
merge mode is GST_TAG_MERGE_REPLACE_ALL:

gst_tag_list_add clears the _whole_ tag list (and not
just the tags I'm trying to add) before adding 
the new tags. OTOH gst_tag_list_add_values doesn't
affect tags already present on tga list that 
are different from the tags I'm trying to add.

IMHO gst_tag_list_add gets it right according to the 
documentation about GST_TAG_MERGE_REPLACE_ALL. 

The difference between the two functions are these lines
at the start of gst_tag_list_add_valist:

  if (mode == GST_TAG_MERGE_REPLACE_ALL) {
    gst_structure_remove_all_fields (list);
  }