swr: update copyright years of a few files iam working on

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: move dither related fields to their own context

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: remove unused variable

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