This ensures that we stay in [0,1], except for plus mode, which
requires a larger and more invasive change.
Bug: skia:
Change-Id: I97f6bcea8b10e70e55ba24bcff759ddbb1761794
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/201460
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>