Similar to existing ADBE Easy Levels2, but provides separate mapping
controls per channel.
Change-Id: Ibc58c58e1e8cb8793d6eb819998c1804ccbbf859
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/268936
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
