In yadif filter, default to top field first if interlacing is unknown

Originally committed as revision 26347 to svn://svn.ffmpeg.org/ffmpeg/trunk

doc/filters.texi

@@ -996,6 +996,8 @@ enable automatic detection
 @end table
 
 Default value is -1.
+If interlacing is unknown or decoder does not export this information,
+top field first will be assumed.
 
 @c man end VIDEO FILTERS
 

libavfilter/vf_yadif.c

@@ -167,7 +167,14 @@ static void return_frame(AVFilterContext *ctx, int is_second)
 {
     YADIFContext *yadif = ctx->priv;
     AVFilterLink *link= ctx->outputs[0];
-    int tff = yadif->parity == -1 ? yadif->cur->video->top_field_first : (yadif->parity^1);
+    int tff;
+
+    if (yadif->parity == -1) {
+        tff = yadif->cur->video->interlaced ?
+            yadif->cur->video->top_field_first : 1;
+    } else {
+        tff = yadif->parity^1;
+    }
 
     if (is_second)
         yadif->out = avfilter_get_video_buffer(link, AV_PERM_WRITE | AV_PERM_PRESERVE |