vc1: warn for unimplemented pan-scan feature.

libavcodec/vc1.c

@@ -863,6 +863,7 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
         }
     }
     if(v->panscanflag) {
+        av_log_missing_feature(v->s.avctx, "Pan-scan", 0);
         //...
     }
     v->rnd = get_bits1(gb);