swscale: use dithering in DITHER_COPY only if not set -sws_dither none

This patch uses dithering in DITHER_COPY macro only if
it was not used option '-sws_dither none'.
With option '-sws_dither none' it uses downshift.

For human eye dithering is OK, for video codecs not necessarily.
If user don't want to use dithering, we should respect that.

Signed-off-by: Mateusz Brzostek <mateuszb@poczta.onet.pl>
Signed-off-by: Michael Niedermayer <michael@niedermayer.cc>