huffyuvenc: add a non-deterministic option

Not actually used in huffyuvenc, but rather in setting the frame
threading.

Example for some files:
context=0:       851974   27226   1137281
context=1,ND=0:  471819   22604    972351
context=1,ND=1:  472875   22673    972582

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>