timecode: add avpriv_timecode_to_string().

libavcodec/timecode.h

@@ -60,6 +60,17 @@ int ff_framenum_to_drop_timecode(int frame_num);
  */
 uint32_t ff_framenum_to_smtpe_timecode(unsigned frame, int fps, int drop);
 
+/**
+ * @brief       Load timecode string in buf
+ * @param buf   Destination buffer
+ * @param tc    Timecode struct pointer
+ * @param frame Frame id (timecode frame is computed with tc->start+frame)
+ * @return a pointer to the buf parameter
+ * @note  buf must have enough space to store the timecode representation
+ *        (sizeof("hh:mm:ss.ff"))
+ */
+char *avpriv_timecode_to_string(char *buf, const struct ff_timecode *tc, unsigned frame);
+
 /**
  * Parse SMTPE 12M time representation (hh:mm:ss[:;.]ff). str and rate fields
  * from tc struct must be set.