swresample: set flags & description and add documentation for output_sample_bits

Allows users to actually use such option.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
lavfi: port sab filter from libmpcodecs
lavfi/mp: remove mp=sab

The filter was ported to a native libavfilter filter.
swr: set scale for 32->32/24 dither

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
swr: dont treat 32 and 24 as equal in simple copy check

Fixes dithering from 32 to 24 bit

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