choosen between 8 and 32 when the supplied parameter is the string
"random".

This is useful for testing the slice support, but it is not supposed
to be used by the user for other purposes and this interface may
change in the future, and thus it is not documented.

The randomization algorithm adopted is the standard Numerical Recipes
LCG.

git-svn-id: file:///var/local/repositories/ffmpeg/trunk@22505 9553f0bf-9b14-0410-a0b8-cfaf0461ba5b