lavf: let av_find_best_stream use bitrate info if available

I guess the user expects to see the stream with the highest bitrate, not with
the most frames, this is especially useful for multi bitrate streams.

This patch changes av_find_best_stream to select the stream based on a number
of conditions, the first condition has the highest priority, the last condition
has the lowest:

1) Select the stream with the highest FFMIN(5, codec_info_nb_frames) value
2) Select the stream with the highest bitrate
3) Select the stream with the highest codec_info_nb_frames
4) Select the first stream

Signed-off-by: Marton Balint <cus@passwd.hu>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -2949,7 +2949,7 @@ int av_find_best_stream(AVFormatContext *ic,
                         int flags)
 {
     int i, nb_streams = ic->nb_streams;
-    int ret = AVERROR_STREAM_NOT_FOUND, best_count = -1;
+    int ret = AVERROR_STREAM_NOT_FOUND, best_count = -1, best_bitrate = -1, best_multiframe = -1, count, bitrate, multiframe;
     unsigned *program = NULL;
     AVCodec *decoder = NULL, *best_decoder = NULL;
 
@@ -2978,9 +2978,16 @@ int av_find_best_stream(AVFormatContext *ic,
                 continue;
             }
         }
-        if (best_count >= st->codec_info_nb_frames)
+        count = st->codec_info_nb_frames;
+        bitrate = avctx->bit_rate;
+        multiframe = FFMIN(5, count);
+        if ((best_multiframe >  multiframe) ||
+            (best_multiframe == multiframe && best_bitrate >  bitrate) ||
+            (best_multiframe == multiframe && best_bitrate == bitrate && best_count >= count))
             continue;
-        best_count = st->codec_info_nb_frames;
+        best_count = count;
+        best_bitrate = bitrate;
+        best_multiframe = multiframe;
         ret = real_stream_index;
         best_decoder = decoder;
         if (program && i == nb_streams - 1 && ret < 0) {