Pass field order flag to libx264

Signed-off-by: Jason Garrett-Glaser <jason@x264.com>
(cherry picked from commit b09f5482)

libavcodec/libx264.c

@@ -105,6 +105,10 @@ static int X264_frame(AVCodecContext *ctx, uint8_t *buf,
             frame->pict_type == FF_P_TYPE ? X264_TYPE_P :
             frame->pict_type == FF_B_TYPE ? X264_TYPE_B :
                                             X264_TYPE_AUTO;
+        if (x4->params.b_tff != frame->top_field_first) {
+            x4->params.b_tff = frame->top_field_first;
+            x264_encoder_reconfig(x4->enc, &x4->params);
+        }
     }
 
     do {