avcodec/vc1: fix decoding of old WMV3 format

The position of the second MV predicitor candidate is slightly different
for the old WMV3 format indicated by RES_RTM_FLAG. This patch fixes
decoding of niceday.wmv on the samples server.

Fixes: #6641
Signed-off-by: Jerome Borsboom <jerome.borsboom@carpalis.nl>

libavcodec/vc1.c

@@ -379,11 +379,6 @@ int ff_vc1_decode_sequence_header(AVCodecContext *avctx, VC1Context *v, GetBitCo
     } else {
         v->res_rtm_flag = get_bits1(gb); //reserved
     }
-    if (!v->res_rtm_flag) {
-        av_log(avctx, AV_LOG_ERROR,
-               "Old WMV3 version detected, some frames may be decoded incorrectly\n");
-        //return -1;
-    }
     //TODO: figure out what they mean (always 0x402F)
     if (!v->res_fasttx)
         skip_bits(gb, 16);

libavcodec/vc1_pred.c

@@ -275,7 +275,10 @@ void ff_vc1_pred_mv(VC1Context *v, int n, int dmv_x, int dmv_y,
         //in 4-MV mode different blocks have different B predictor position
         switch (n) {
         case 0:
-            off = (s->mb_x > 0) ? -1 : 1;
+            if (v->res_rtm_flag)
+                off = s->mb_x ? -1 : 1;
+            else
+                off = s->mb_x ? -1 : 2 * s->mb_width - wrap - 1;
             break;
         case 1:
             off = (s->mb_x == (s->mb_width - 1)) ? -1 : 1;