Extend DV autodetection to also reliably detect single-frame DVs with

a higher score that MAX/4.
It checks that there are at least 10 DIF headers and at least one per
24000 bytes, and if so considers the file reliably detected as DV.
Passes probetest, too.

Originally committed as revision 20074 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavformat/dv.c

@@ -489,6 +489,7 @@ static int dv_probe(AVProbeData *p)
     unsigned state, marker_pos = 0;
     int i;
     int matches = 0;
+    int secondary_matches = 0;
 
     if (p->buf_size < 5)
         return 0;
@@ -497,6 +498,10 @@ static int dv_probe(AVProbeData *p)
     for (i = 4; i < p->buf_size; i++) {
         if ((state & 0xffffff7f) == 0x1f07003f)
             matches++;
+        // any section header, also with seq/chan num != 0,
+        // should appear around every 12000 bytes, at least 10 per frame
+        if ((state & 0xff07ff7f) == 0x1f07003f)
+            secondary_matches++;
         if (state == 0x003f0700 || state == 0xff3f0700)
             marker_pos = i;
         if (state == 0xff3f0701 && i - marker_pos == 80)
@@ -505,7 +510,7 @@ static int dv_probe(AVProbeData *p)
     }
 
     if (matches && p->buf_size / matches < 1024*1024) {
-        if (matches > 4)
+        if (matches > 4 || (secondary_matches >= 10 && p->buf_size / secondary_matches < 24000))
             return AVPROBE_SCORE_MAX*3/4; // not max to avoid dv in mov to match
         return AVPROBE_SCORE_MAX/4;
     }