Pass alphaType to append_gamut_transform() to inform the clamp

BUG=skia:

Change-Id: I1a8aef36043d4091bffae95b0275fa7fa8a35c97
Reviewed-on: https://skia-review.googlesource.com/9441
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Klein <mtklein@chromium.org>
3 files changed