In V1 we received complaints that Renderer was too locked down.
In particular, it wasn't possible to implement a Renderer that
wrapped another Renderer, since most of the methods were package
private and final (e.g. enable).
This change defines Renderer as a proper interface, removing
this limitation. A method-reordering-only change will follow
this one, to get things into a nice consistent/sane order in
each of the Renderer classes.
-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=127527995
