lavfi/blend: fix SW/SH calculation

Signed-off-by: Paul B Mahol <onemda@gmail.com>
vf_fade: Fix alpha flag integration

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
vf_fade: Support slice threaded alpha

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