Add Regression Test for D3D11 Alpha Passthrough As a follow up to commit "8d6af08 Use dedicated ALPHA passthrough shader", this change adds a regression test that verifies RGB channels are set to zero when performing an ALPHA copy. BUG=angleproject:2239 Change-Id: Icc08deb523c388a0f2cbdcadd1eec69a97fbe136 Reviewed-on: https://chromium-review.googlesource.com/774579 Commit-Queue: Jamie Madill <jmadill@chromium.org> Reviewed-by: Jamie Madill <jmadill@chromium.org>