timecode: move dropframe code and doxycomment it.

This is based on the original work by Baptiste Coudurier.

libavcodec/timecode.h

@@ -43,6 +43,14 @@ struct ff_timecode {
     AVRational rate; ///< Frame rate in rationnal form
 };
 
+/**
+ * @brief           Adjust frame number for NTSC drop frame time code
+ * @param frame_num Actual frame number to adjust
+ * @return          Adjusted frame number
+ * @warning         Adjustment is only valid in NTSC 29.97
+ */
+int ff_framenum_to_drop_timecode(int frame_num);
+
 /**
  * Parse SMTPE 12M time representation (hh:mm:ss[:;.]ff). str and rate fields
  * from tc struct must be set.