plug-ins: fix #6258 Hierarchical XMP tag should be written to Array

When saving XMP metadata were using gexiv2_metadata_get_tag_string for all
tags, even those that can have multiple values. This caused those values
to be saved as one value instead of multiple.

To fix this we use gexiv2_metadata_get_tag_multiple, except for XmpText.
Then we add all returned values for that tag separately to our metadata.

(cherry picked from commit af888e481f)

libgimpbase/gimpmetadata.c

@@ -822,6 +822,11 @@ gimp_metadata_serialize (GimpMetadata *metadata)
   if (xmp_data)
     {
       for (i = 0; xmp_data[i] != NULL; i++)
+        {
+          /* XmpText is always a single value, but structures like
+           * XmpBag and XmpSeq can have multiple values that need to be
+           * treated separately or else saving will do things wrong. */
+          if (! g_strcmp0 (gexiv2_metadata_get_tag_type (xmp_data[i]), "XmpText"))
             {
               value = gexiv2_metadata_get_tag_string (GEXIV2_METADATA (metadata),
                                                       xmp_data[i]);
@@ -830,7 +835,27 @@ gimp_metadata_serialize (GimpMetadata *metadata)
 
               gimp_metadata_append_tag (string, xmp_data[i], escaped, base64);
             }
+          else
+            {
+              gchar **values;
 
+              values = gexiv2_metadata_get_tag_multiple (GEXIV2_METADATA (metadata),
+                                                         xmp_data[i]);
+
+              if (values)
+                {
+                  gint  vi;
+
+                  for (vi = 0; values[vi] != NULL; vi++)
+                    {
+                      escaped = gimp_metadata_escape (xmp_data[i], values[vi], &base64);
+                      gimp_metadata_append_tag (string, xmp_data[i], escaped, base64);
+                    }
+
+                  g_strfreev (values);
+                }
+            }
+        }
       g_strfreev (xmp_data);
     }