mpeg4: dont reset picture_num for xvid

Fixes Ticket1162

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
(cherry picked from commit a4e359a3f9)

libavcodec/h263dec.c

@@ -571,7 +571,6 @@ retry:
     if (s->codec_id == CODEC_ID_MPEG4 && s->xvid_build>=0 && avctx->idct_algo == FF_IDCT_AUTO && (av_get_cpu_flags() & AV_CPU_FLAG_MMX)) {
         avctx->idct_algo= FF_IDCT_XVIDMMX;
         ff_dct_common_init(s);
-        s->picture_number=0;
     }
 #endif