dxva2_vc1: include the start code in wMBbitOffset

This resolves a decoding failure on Intel GPUs.
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/dxva2_vc1.c

@@ -168,7 +168,7 @@ static void fill_slice(AVCodecContext *avctx, DXVA_SliceInfo *slice,
     slice->dwSliceDataLocation = position;
     slice->bStartCodeBitOffset = 0;
     slice->bReservedBits       = (s->pict_type == AV_PICTURE_TYPE_B && !v->bi_type) ? v->bfraction_lut_index + 9 : 0;
-    slice->wMBbitOffset        = v->p_frame_skipped ? 0xffff : get_bits_count(&s->gb);
+    slice->wMBbitOffset        = v->p_frame_skipped ? 0xffff : get_bits_count(&s->gb) + (avctx->codec_id == AV_CODEC_ID_VC1 ? 32 : 0);
     slice->wNumberMBsInSlice   = s->mb_width * s->mb_height; /* XXX We assume 1 slice */
     slice->wQuantizerScaleCode = v->pq;
     slice->wBadSliceChopping   = 0;