Simplify tff setting code in MPV_frame_start().

Originally committed as revision 26090 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -964,13 +964,11 @@ int MPV_frame_start(MpegEncContext *s, AVCodecContext *avctx)
 
         s->current_picture_ptr= 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
+        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->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;
     }