sws: add dither enum

This allows specifying more dither algorithms without using up flags and
without ambiguities.

Also initialize the new field based on the flags and use it.
Note, improving the logic of the checks is left to subsequent
commits, this here only switches from flags to enum.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
avfilter/vf_scale: generic swscale option support

With this all AVOptions from swscale can be set without each needing
changes to vf_scale.c

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