Dithering relies on a 8 line dithering table and the code always uses it from
the beginning. So in order to make dithering independent from height of the
slices used we must enforce a 8 line alignment.
Fixes issue #20071.
Signed-off-by: Marton Balint <cus@passwd.hu>
(cherry picked from commit b61e510e75)
057b94649f