We don't need the ending beat until the change has already started, so
just use the legitimate On to calculate things rather than doing
anything hacky.
We load foreground changes at the beginning of the screen, but they
shouldn't be issued an OnCommand until it's time for them to start. The
Foreground layer, however, should get the OnCommand right away. So
don't propagate it, since we already issue On manually to changes.
Devin J. Pohly