SkNamedTransferFn: Change kRec709 to match ITU-R BT.1886 pure 2.4 gamma

Bug: b/230400473
Bug: 40539111
Change-Id: I51706c5a2861d0cda0858b4e4f44c1ca72184062
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/1048477
Reviewed-by: Florin Malita <fmalita@google.com>
Reviewed-by: Kaylee Lubick <kjlubick@google.com>
Commit-Queue: Christopher Cameron <ccameron@google.com>
2 files changed