This helper tool in Renderer11 can be useful in other back-ends,
or even in the front end.

BUG=angleproject:1635

Change-Id: I18ec19a891a9bdfa7b80dea1b8e308abf206906b
Reviewed-on: https://chromium-review.googlesource.com/450919
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Corentin Wallez <cwallez@chromium.org>
Reviewed-by: Geoff Lang <geofflang@chromium.org>