AVcodec: limit the number of auto-detected threads to 4 by default

Revert this when you have found the reason why it fails for soo many
people, in avi, mp4 (and mkv before the HACK)
(cherry picked from commit ce85748b)
Signed-off-by: Jean-Baptiste Kempf <jb@videolan.org>

modules/codec/avcodec/video.c

@@ -341,6 +341,9 @@ int InitVideoDec( decoder_t *p_dec, AVCodecContext *p_context,
         i_thread_count = vlc_GetCPUCount();
         if( i_thread_count > 1 )
             i_thread_count++;
+
+        //FIXME: take in count the decoding time
+        i_thread_count = __MIN( i_thread_count, 4 );
     }
     i_thread_count = __MIN( i_thread_count, 16 );
     msg_Dbg( p_dec, "allowing %d thread(s) for decoding", i_thread_count );