Refactor as a single interpolating loop, based on careful selection
of lerp coefficients.
Change-Id: I58786cddb2f042b53dcbac80c2346736429be102
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/252858
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
