Made Display animated (as opposed to reactive).

In order to prevent choppy/unstable fluxuation in state, services like
Display wait for their inputs to stabilize before mutation.

Clients can still prescribe future activity by supplying values like
functions of time, that refer to live signals.

That is, while the set of videos Display renders only changes when
inputs stabilize, any video can be a function like

    function(time) {
      return SomePicture(someSignal(time));
    }

Not only is the video still changing due to the time parameter, but it's
changing in response to live application state by referring to a signal.
So animation isn't as big of a blow as it might seem at first glance.

Intuitively I feel this is the right approach. I admit I'll need to test
it on nontrivial applications in order to be sure things are better this
way.