Most of this is just tidying up the logic to (hopefully) be better to
follow, again (again (again)).
The actual fix is that we now allow interpolation/playback when the
incoming time is less than the first frame's time, regardless of
receiving status.
The parent clock will not unpause until WaitingForFrames becomes false,
so I've moved the set of that before we start to propagate its values
across. Doesn't fix any visible issue but should make propagation one
game loop faster.