avformat/utils: Redesign scoring in av_find_default_stream_index()

This avoids empty streams from being selected if a equivalent non empty one is available
The new system is also clearer and more systematic
This may need finetuning, which should be easy to do ...

Fixes Ticket2687
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavformat/utils.c

@@ -1585,26 +1585,26 @@ int av_find_default_stream_index(AVFormatContext *s)
     int i;
     AVStream *st;
     int best_stream = 0;
-    int best_score = -1;
+    int best_score = INT_MIN;
 
     if (s->nb_streams <= 0)
         return -1;
     for (i = 0; i < s->nb_streams; i++) {
         int score = 0;
         st = s->streams[i];
-        if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO &&
-            !(st->disposition & AV_DISPOSITION_ATTACHED_PIC)) {
-            if (!st->codec->width && !st->codec->height && !st->codec_info_nb_frames)
-                score += 25;
-            else
-                score += 100;
+        if (st->codec->codec_type == AVMEDIA_TYPE_VIDEO) {
+            if (st->disposition & AV_DISPOSITION_ATTACHED_PIC)
+                score -= 400;
+            if (st->codec->width && st->codec->height)
+                score += 50;
+            score+= 25;
         }
         if (st->codec->codec_type == AVMEDIA_TYPE_AUDIO) {
-            if (!st->codec->sample_rate && !st->codec_info_nb_frames)
-                score += 12;
-            else
+            if (st->codec->sample_rate)
                 score += 50;
         }
+        if (st->codec_info_nb_frames)
+            score += 12;
 
         if (st->discard != AVDISCARD_ALL)
             score += 200;