Fix deblocking filter for field pictures by properly looking for first

row in bottom fields and preventing the use of "fast" deblocking for such.

Originally committed as revision 10950 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -6371,12 +6371,13 @@ static void filter_mb_edgech( H264Context *h, uint8_t *pix, int stride, int16_t
 
 static void filter_mb_fast( H264Context *h, int mb_x, int mb_y, uint8_t *img_y, uint8_t *img_cb, uint8_t *img_cr, unsigned int linesize, unsigned int uvlinesize) {
     MpegEncContext * const s = &h->s;
+    int mb_y_firstrow = s->picture_structure == PICT_BOTTOM_FIELD;
     int mb_xy, mb_type;
     int qp, qp0, qp1, qpc, qpc0, qpc1, qp_thresh;
 
     mb_xy = mb_x + mb_y*s->mb_stride;
 
-    if(mb_x==0 || mb_y==0 || !s->dsp.h264_loop_filter_strength || h->pps.chroma_qp_diff ||
+    if(mb_x==0 || mb_y==mb_y_firstrow || !s->dsp.h264_loop_filter_strength || h->pps.chroma_qp_diff ||
        (h->deblocking_filter == 2 && (h->slice_table[mb_xy] != h->slice_table[h->top_mb_xy] ||
                                       h->slice_table[mb_xy] != h->slice_table[mb_xy - 1]))) {
         filter_mb(h, mb_x, mb_y, img_y, img_cb, img_cr, linesize, uvlinesize);