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