[skottie] Add tritone layer effect support

Generalize sksg::TintColorFilter into GradientColorfilter, to support
multiple color interpolation ranges.

Keep the two-color/single-matrix (tint) specialization, and use a table
color filter for the general case.

Change-Id: Idf42833462127c29fb69fe94337a1d1478b77dd9
Reviewed-on: https://skia-review.googlesource.com/c/194024
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
3 files changed