Make transfer function classification part of the public API

Change-Id: I9580a89ab6211b411c3e741b16cd6d0980b806b2
Reviewed-on: https://skia-review.googlesource.com/c/skcms/+/616520
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed