parse video frame headers to get key flag

Originally committed as revision 7104 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/mxf.c

@@ -896,6 +896,7 @@ static int mxf_parse_structural_metadata(MXFContext *mxf)
             st->codec->width = descriptor->width;
             st->codec->height = descriptor->height;
             st->codec->bits_per_sample = descriptor->bits_per_sample; /* Uncompressed */
+            st->need_parsing = 2; /* only parse headers */
         } else if (st->codec->codec_type == CODEC_TYPE_AUDIO) {
             container_ul = mxf_get_codec_ul(mxf_sound_essence_container_uls, &descriptor->essence_container_ul);
             if (st->codec->codec_id == CODEC_ID_NONE)