disable vp3 mmx idct for theora files to avoid artifacts

(see theora-a4_v6-k250-s0_2.ogg)

Originally committed as revision 6253 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/i386/dsputil_mmx.c

@@ -3056,6 +3056,7 @@ void dsputil_init_mmx(DSPContext* c, AVCodecContext *avctx)
                 c->idct_permutation_type= FF_LIBMPEG2_IDCT_PERM;
 #endif
             }else if(idct_algo==FF_IDCT_VP3 &&
+                     avctx->codec->id!=CODEC_ID_THEORA &&
                      !(avctx->flags & CODEC_FLAG_BITEXACT)){
                 if(mm_flags & MM_SSE2){
                     c->idct_put= ff_vp3_idct_put_sse2;