Clamp parametric gamma values to 0-1 range

BUG=skia:

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4641

Change-Id: I1c6963da956500deea559d5ece31529add89980a
Reviewed-on: https://skia-review.googlesource.com/4641
Reviewed-by: Robert Aftias <raftias@google.com>
Commit-Queue: Matt Sarett <msarett@google.com>
1 file changed