lavfi/afade: add timeline support

For correct precision one may need to use asetnsamples.

Signed-off-by: Paul B Mahol <onemda@gmail.com>
av_d2q: Fix infinity check

The old check would fail on huge but not infinite values
and the later code could then fail to handle them correctly in
some cases.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>
av_d2q: fix rounding for negative values

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