lavf: add mdec to is_intra_only() list

Signed-off-by: Mans Rullgard <mans@mansr.com>

libavformat/utils.c

@@ -791,6 +791,7 @@ static int is_intra_only(AVCodecContext *enc){
         case CODEC_ID_VCR1:
         case CODEC_ID_DNXHD:
         case CODEC_ID_JPEG2000:
+        case CODEC_ID_MDEC:
             return 1;
         default: break;
         }