The fix is in the mix().
Bug: tint:1429
Change-Id: Id128ead6b124cd364f45a860ad991977be94be5b
Reviewed-on: https://dawn-review.googlesource.com/c/tint/+/80660
Kokoro: Kokoro <noreply+kokoro@google.com>
Reviewed-by: Ben Clayton <bclayton@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>