Extend the slice filter to make it issue slice height values randomly

Extend the slice filter to make it issue slice height values randomly
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 for other purposes and this interface may change in the
future, thus it is not documented.

The randomization algorithm adopted is the standard Numerical Recipes
LCG.

Originally committed as revision 22505 to svn://svn.ffmpeg.org/ffmpeg/trunk