lavf: consider codec framerate for framerate detection

Fixes detection of some TV sample as 24.5 FPS. With the patch applied,
it's detected as 25 FPS.

This is enabled for mpegts only.

libavformat/internal.h

@@ -145,6 +145,11 @@ struct AVFormatInternal {
      * ID3v2 tag useful for MP3 demuxing
      */
     AVDictionary *id3v2_meta;
+
+    /*
+     * Prefer the codec framerate for avg_frame_rate computation.
+     */
+    int prefer_codec_framerate;
 };
 
 struct AVStreamInternal {