Add clamped/unclamped xform to GrColorSpaceXform

Use that in analytic gradient setup for future-proofing. Also fixed
several out-of-date comments in gradient code.

Bug: skia:
Change-Id: I79726cad786c22f80e08cdc2b7a1e15ae27ecd5a
Reviewed-on: https://skia-review.googlesource.com/62320
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Brian Osman <brianosman@google.com>
6 files changed