h264: Fix ticket #3147 H264 - Wrong field order

Default field order to top field first when interlaced frame is detected and pic_struct_present_flag is not set.
Since bottom field first comes from the old NTSC standard and is not used with HD anymore I think it´s straight forward to favor the majority.
Signed-off-by: Thomas Mundt <loudmax@yahoo.de>
Reviewed-by: Kieran Kunhya <kierank@obe.tv>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>

libavcodec/h264.c

@@ -803,7 +803,7 @@ static void decode_postinit(H264Context *h, int setup_finished)
         /* Derive top_field_first from field pocs. */
         cur->f->top_field_first = cur->field_poc[0] < cur->field_poc[1];
     } else {
-        if (cur->f->interlaced_frame || h->sps.pic_struct_present_flag) {
+        if (h->sps.pic_struct_present_flag) {
             /* Use picture timing SEI information. Even if it is a
              * information of a past frame, better than nothing. */
             if (h->sei_pic_struct == SEI_PIC_STRUCT_TOP_BOTTOM ||
@@ -811,6 +811,10 @@ static void decode_postinit(H264Context *h, int setup_finished)
                 cur->f->top_field_first = 1;
             else
                 cur->f->top_field_first = 0;
+        } else if (cur->f->interlaced_frame) {
+            /* Default to top field first when pic_struct_present_flag
+             * is not set but interlaced frame detected */
+            cur->f->top_field_first = 1;
         } else {
             /* Most likely progressive */
             cur->f->top_field_first = 0;