Port (or Tracks) can be safely added during playback, however
the disk-reader's playback buffer is initially empty. This lead to
false-positive Underrun() signals when processing takes place
before or concurrently with re-filling the disk-buffer for the new
channels.
Now new empty buffers are ignored, and produce silence until the
initial refill is complete. There is however no per-channel
de-click in, yet.
This fixes: play some audio track, ctrl+drag a region to the
drop-zone, creating a new track while playing.
648beb94a0