... The UI will still be present, but won't work

... maybe changing how non-LTI stereo effects (reverbs?) apply to mono tracks

... Also possible slight change in graphical meter level as explained in
comments

... because that atomic might otherwise change

... the other playback policy subclasses (scrub, cut preview, looping) were
not used for recording; and recording never went backwards

... Process both pieces of the intermediate result first (when it is not
contiguous), before doing any of the shifting of samples.

This bug might have happened only on some passes of TransformPlayBuffers.

... track states do not yet vary independently, but this prepares for a better
treatment of differing effect latencies on tracks.

... and can be done before the update of the time queue, so long as the flushing
of the ring buffers is later.

... letting playback states and mixers progress independently when the
non-destructive effect latencies differ; and flushing only once.

RingBuffer::Excess was needed transitionally, but is now unused.  Maybe it will
find another use again.