Print error message when true interlaced VC-1 frames are encountered

to inform user why decoder produces no output.

Originally committed as revision 20196 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/vc1.c

@@ -781,7 +781,11 @@ int vc1_parse_frame_header_adv(VC1Context *v, GetBitContext* gb)
 
     if(v->interlace){
         v->fcm = decode012(gb);
-        if(v->fcm) return -1; // interlaced frames/fields are not implemented
+        if(v->fcm){
+            if(!v->warn_interlaced++)
+                av_log(v->s.avctx, AV_LOG_ERROR, "Interlaced frames/fields support is not implemented\n");
+            return -1;
+        }
     }
     switch(get_unary(gb, 0, 4)) {
     case 0:

libavcodec/vc1.h

@@ -312,6 +312,8 @@ typedef struct VC1Context{
     uint8_t closed_entry;       ///< Closed entry point flag (CLOSED_ENTRY syntax element)
 
     int parse_only;             ///< Context is used within parser
+
+    int warn_interlaced;
 } VC1Context;
 
 /** Find VC-1 marker in buffer