avfilter/vf_rotate: support slice threading

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avfilter/vf_hflip: refactor plane dimensions calculation

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avfilter/vf_hflip: support slice threading

Signed-off-by: Paul B Mahol <onemda@gmail.com>
doc: try to add a link from the vf_scale sws_flags to the scaler docs

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