Added onRender callbacks to <Renderer> and <TeX>.

Summary:
These are used by the Sorter and Matcher to know when their contents
have changed and widths/heights need to be resynced.

Test Plan: Callbacks fired properly in Sorter and Matcher.

Reviewers: jack

Reviewed By: jack

CC: alpert

Differential Revision: http://phabricator.khanacademy.org/D6056