started quickly, queued position data from the previous play will still
be flushed. We clear already-flushed data in StopMixing (pos_map.clear())
so we don't have stale data, but some is being re-flushed afterwards because
we have the same ID. Change sound IDs when stopping a sound.
(The symptom was the first frame in Play mode in the editor would process
using an old position; the position was usually later in the song, which
sometimes marked hold notes as dead.)
Glenn Maynard
712e1ec445