warmup, remove clamping in append_gamut_transform()

Clamping here seems inconsistent with our color pipeline model,
and with the existing GPU impl.  The SkRasterPipeline store
stages already do clamp when storing unorms, and table-lookup
stages clamp their inputs, so it should be safe.

While refactoring, slim its interface down a bit.

Change-Id: I4772457fdf90e483834d034f02974d7a859cbe24
Reviewed-on: https://skia-review.googlesource.com/130902
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed