Add skcms_BestSingleCurve utility

This simplifies the task of picking a single transfer function for
rasterization to calling a single function. Returns the best parametric
(or approximated) TRC curve, or sRGB when all else fails.

Bug: skia:
Change-Id: I2417be0425020d45de6a12ab6000feafe690951e
Reviewed-on: https://skia-review.googlesource.com/121561
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Mike Klein <mtklein@chromium.org>
42 files changed