lavc/h264: create AVFrame side data from H.264 timecodes

Create SMPTE ST 12-1 timecodes based on H.264 SEI picture timing
info.

For framerates > 30 FPS, the field flag is used in conjunction with
pairs of frames which contain the same frame timestamp in S12M.
Ensure the field is properly set per the spec.

libavutil/frame.h

@@ -158,6 +158,14 @@ enum AVFrameSideDataType {
      */
     AV_FRAME_DATA_QP_TABLE_DATA,
 #endif
+
+    /**
+     * Timecode which conforms to SMPTE ST 12-1. The data is an array of 4 uint32_t
+     * where the first uint32_t describes how many (1-3) of the other timecodes are used.
+     * The timecode format is described in the av_timecode_get_smpte_from_framenum()
+     * function in libavutil/timecode.c.
+     */
+    AV_FRAME_DATA_S12M_TIMECODE,
 };
 
 enum AVActiveFormatDescription {