swscale: fix crash with dithering due incorrect offset calculation.

ptrdiff_t can be 4 bytes, which leads to the next element being 4-byte
aligned and thus at a different offset than intended. Forcing 8-byte
alignment forces equal offset of dither16/32 on x86-32 and x86-64.

Signed-off-by: Ronald S. Bultje <rsbultje@gmail.com>