Fix alpha coverage for lerp_565 stage.

Like three gray with alpha blends and taking the max alpha.

Change-Id: I104c84f784979030744127f8f66905ad9d1bdf0e
Reviewed-on: https://skia-review.googlesource.com/15898
Commit-Queue: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
4 files changed