This works around a bug in the AMD driver that writes 0's to the
first attachment if it isn't written by the pixel shader.

BUG=angleproject:2048

Change-Id: I384fd60c0e0a37fbc0fd7b69fe1ec74fe4ffac8f
Reviewed-on: https://chromium-review.googlesource.com/531630Reviewed-by: Jamie Madill <jmadill@chromium.org>
Commit-Queue: Corentin Wallez <cwallez@chromium.org>