added skip macroblock optimization (big perf win on black regions for example)

Originally committed as revision 13 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/msmpeg4.c

@@ -694,6 +694,7 @@ int msmpeg4_decode_mb(MpegEncContext *s,
                 s->mv_type = MV_TYPE_16X16;
                 s->mv[0][0][0] = 0;
                 s->mv[0][0][1] = 0;
+                s->mb_skiped = 1;
                 return 0;
             }
         }