This benign warning would show up in the Debug layer messages. It was
causing the test to fail in some standalone configurations.

This fix implemented here is to keep 8 different pixel shaders, one
for each total count of render targets, up to the implementation max.

BUG=angleproject:2025

Change-Id: I826ddae686ddb1cfad17879644152be2f6125c5b
Reviewed-on: https://chromium-review.googlesource.com/617512Reviewed-by: Geoff Lang <geofflang@chromium.org>
Commit-Queue: Jamie Madill <jmadill@chromium.org>