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

jomanmuk/vlc-2.2,jomanmuk/vlc-2.1,krichter722/vlc,vlc-mirror/vlc,vlc-mirror/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,vlc-mirror/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.2,krichter722/vlc,krichter722/vlc,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.1,vlc-mirror/vlc-2.1,jomanmuk/vlc-2.1,xkfz007/vlc,xkfz007/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,krichter722/vlc,shyamalschandra/vlc,shyamalschandra/vlc,jomanmuk/vlc-2.2,xkfz007/vlc,xkfz007/vlc,krichter722/vlc,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,vlc-mirror/vlc-2.1,xkfz007/vlc,krichter722/vlc,jomanmuk/vlc-2.1,vlc-mirror/vlc,shyamalschandra/vlc,shyamalschandra/vlc,vlc-mirror/vlc,vlc-mirror/vlc-2.1,xkfz007/vlc,jomanmuk/vlc-2.1,jomanmuk/vlc-2.2,jomanmuk/vlc-2.2