avcodec/apng: Add partial support for blending with PAL8 pixel format

Currently restricted to blending pixels that only contain either
0 or 255 in their alpha components

Signed-off-by: Donny Yang <work@kota.moe>
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>