Revert renderer props to editor.toJSON

Summary:
Reverts a small change in D5384, which sent editor props
to the renderer instead of the result of editor.toJSON

We decided offline that this may be desirable long term,
but to minimize changes and temporarily unbreak the
transformer, we're reverting it for now, and will
revisit this decision once we've audited the use
of props and toJSON in all the various widgets.

Test Plan:
 * try the various widgets and hope they still work.
 * transformers were definitely broken before, so make sure
   that the correct shape is drawn in gray in the correct
   position for a transformer.

Auditors: alex, joel