Normally, a Laplace distribution is more typical of the residuals
encoded, but for noisy input, it's both better and simpler to be
safe and use a 1/d^2 distribution. Second hunk could use some
renormalization but it has effectively little impact.

Output size of ffvhuff on various 4:2:0 sequences:
context=0,1/d:    851974   27226   1137281
context=0,1/d²:   619081   25069   1051500
context=0,1/d³:   501983   30454   1290561
context=0,lapl:   500650   31754   1304082
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>