Reference article in favour of one big triangle instead of 2:
https://michaldrobot.com/2014/04/01/gcn-execution-patterns-in-full-screen-passes/

According to this article, the performance could be increased by ~10% for fullscreen shaders.

Bug: angleproject:2634
Change-Id: Ia5b04c40f0587e3cb8680c0f30f7b68d9d7a3efe
Reviewed-on: https://chromium-review.googlesource.com/c/angle/angle/+/2193192
Commit-Queue: Jamie Madill <jmadill@chromium.org>
Reviewed-by: Jamie Madill <jmadill@chromium.org>