add a clipped textblock proxy test

Summary:
this renders a classic text passage split in twain between a real textblock
and a proxy render of the textblock to mimic rendering only a textblock

also provides a define for easily testing against raw textblock render perf
Depends on D8877

Reviewers: cedric, Hermet

Reviewed By: Hermet

Subscribers: Hermet

Tags: expedite

Differential Revision: https://phab.enlightenment.org/D8878