Adjust threshold for xvid packed b frame detection.

Also add xvid_build check.
If you note any packed bitstream regression, check this commit first and report it.
Fixes issue1652

Originally committed as revision 20997 to svn://svn.ffmpeg.org/ffmpeg/trunk

libavcodec/h263dec.c

@@ -682,7 +682,7 @@ retry:
                 }
             }
         }
-        if(s->gb.buffer == s->bitstream_buffer && buf_size>20){ //xvid style
+        if(s->gb.buffer == s->bitstream_buffer && buf_size>7 && s->xvid_build){ //xvid style
             startcode_found=1;
             current_pos=0;
         }