Fixed mpeg12 top field first flag value with field picture encoding.

The relevent extract of the iso 13818-2 about the value of the syntaxical
element top_field_first of the Picture Coding Extension is:

"top_field_first -- The meaning of this element depends upon picture_structure,
progressive_sequence and repeat_first_field.
[...]
In a field picture top_field_first shall have the value '0', and the only field
output by the decoding process is the decoded field picture."

Originally committed as revision 24853 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -963,7 +963,14 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
             return -1;
 
         s->current_picture_ptr= pic;
-        s->current_picture_ptr->top_field_first= s->top_field_first; //FIXME use only the vars from current_pic
+        //FIXME use only the vars from current_pic
+        if(s->codec_id == CODEC_ID_MPEG1VIDEO || s->codec_id == CODEC_ID_MPEG2VIDEO) {
+            if(s->picture_structure == PICT_FRAME)
+                s->current_picture_ptr->top_field_first= s->top_field_first;
+            else
+                s->current_picture_ptr->top_field_first= (s->picture_structure == PICT_TOP_FIELD) == s->first_field;
+        } else
+            s->current_picture_ptr->top_field_first= s->top_field_first;
         s->current_picture_ptr->interlaced_frame= !s->progressive_frame && !s->progressive_sequence;
     }