doc/filters: mention configure option for ladspa

Signed-off-by: Paul B Mahol <onemda@gmail.com>
avcodec/avcodec.h: skip_alpha should not be accessed directly (would cause ABI issues)

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
tests/fate/prores: fix skip alpha tests on big endian

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