avformat: factorize iso 8601 timestamp writer to a dictionary avutil function

Signed-off-by: Marton Balint <cus@passwd.hu>

libavutil/internal.h

@@ -330,6 +330,16 @@ static av_always_inline av_const int avpriv_mirror(int x, int w)
 
 void ff_check_pixfmt_descriptors(void);
 
+/**
+ * Set a dictionary value to an ISO-8601 compliant timestamp string.
+ *
+ * @param s AVFormatContext
+ * @param key metadata key
+ * @param timestamp unix timestamp in microseconds
+ * @return <0 on error
+ */
+int avpriv_dict_set_timestamp(AVDictionary **dict, const char *key, int64_t timestamp);
+
 extern const uint8_t ff_reverse[256];
 
 #endif /* AVUTIL_INTERNAL_H */