ad_pcm: Track pts explicitly

ad_pcm: Track pts explicitly

ad_pcm used the old audio timestamp tracking system that calculated
timestamp at end of decoder output as
last_timestamp_in_input_decoder_has_read +
bytes_read_after_that_timestamp / input_bitrate.
For PCM this can be accurate as input bitrate is constant. However it
relies on input bitrate being known and actually set. At least in some
case with .mov input and libavformat demuxer it wasn't set. Instead of
special-casing PCM to make sure input bitrate is set (in general it
may not be known or constant at all) change ad_pcm to explicitly set
the pts information on the decoder output side.