Bounces passed no bwf to the encoder, so mix and stem WAVs had no bext
chunk and post tools placed them at zero. Bounce::new now stamps them
with TimeReference = the session's timecode start plus the bounce start,
in samples since midnight, as other DAWs do. A caller's own bwf wins;
AIFF and FLAC have no bext and are unchanged.
Fixes#1296.