Enforcing that _computeLayout must be called before _render

For @tkimcoop , I think is mostly regression tests aimed at animations.

Fixes #867
What it used to be was somewhat essentially a hackhack so that the plot scales don't use undefined in their ranges, since plots would render before computeLayout was called.  Now I want to make the following assertions:  render can only happen after computeLayout is called, and a component is not given space until computeLayout has been computed.