add SkNamedTransferFn::kRec2020

Change-Id: I674ab82845f5e6db56412d36e2d146d1208032af
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/215824
Reviewed-by: Mike Klein <mtklein@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
4 files changed