glib-backports: Add a note that a couple of functions aren't backports

We can't backport GLib's implementations of these, because they make use
of GHashTable/GPtrArray internals. Instead, we have a reimplementation
of the same API, which accepts the cost of some redundant work as a
reasonable price to pay for backwards-compatibility.

Signed-off-by: Simon McVittie <smcv@collabora.com>

common/flatpak-glib-backports-private.h

@@ -117,6 +117,9 @@ g_date_time_new_from_iso8601 (const gchar *text, GTimeZone *default_tz)
 #endif
 
 #if !GLIB_CHECK_VERSION (2, 58, 0)
+/* This is a reimplementation rather than a backport, and is a little less
+ * efficient than the real g_hash_table_steal_extended(), since it can't
+ * see into GHashTable internals */
 static inline gboolean
 g_hash_table_steal_extended (GHashTable    *hash_table,
                              gconstpointer  lookup_key,

common/flatpak-glib-backports.c

@@ -697,6 +697,8 @@ g_get_language_names_with_category (const gchar *category_name)
 #endif
 
 #if !GLIB_CHECK_VERSION (2, 62, 0)
+/* This is a reimplementation, not a backport: the version in GLib makes
+ * use of GPtrArray internals */
 void
 g_ptr_array_extend (GPtrArray  *array_to_extend,
                     GPtrArray  *array,