avcodec/decode: set best_effort_timestamp on output frames for all decoders

avcodec/decode: set best_effort_timestamp on output frames for all decoders

Fixes a decoding regression introduced by e9a2a87773, and as a side effect also
fixes bogus values set to certain audio frames that had some samples discarded,
where the offsets added to pts, pkt_dts and pkt_duration were not reflected in
best_effort_timestamp.

Signed-off-by: James Almer <jamrial@gmail.com>