Do not assume PIX_FMT_YUV420P will remain the only supported output format for H264.

Originally committed as revision 25352 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h264.c

@@ -1917,7 +1917,7 @@ static int decode_slice_header(H264Context *h, H264Context *h0){
                 if (prev) {
                     av_image_copy(h->short_ref[0]->data, h->short_ref[0]->linesize,
                                   (const uint8_t**)prev->data, prev->linesize,
-                                  PIX_FMT_YUV420P, s->mb_width*16, s->mb_height*16);
+                                  s->avctx->pix_fmt, s->mb_width*16, s->mb_height*16);
                     h->short_ref[0]->poc = prev->poc+2;
                 }
                 h->short_ref[0]->frame_num = h->prev_frame_num;