skip blocks with small chroma dc too (if the user wants it) needed to avoid...

skip blocks with small chroma dc too (if the user wants it) needed to avoid IDCT(input coeffs !=0) == 0 problems which cause catastrophic error accumulation at qp=1

Originally committed as revision 853 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/mpegvideo.c

@@ -1589,7 +1589,11 @@ static inline void dct_single_coeff_elimination(MpegEncContext *s, int n, int th
     const int last_index= s->block_last_index[n];
 
     if(skip_dc) skip_dc=1;
-    
+    if(threshold<0){
+        skip_dc=0;
+        threshold= -threshold;
+    }
+
     /* are all which we could set to zero are allready zero? */
     if(last_index<=skip_dc - 1) return;