Harden linear gradients

Ensure that the last offset == 1.0f when implict positions are used
(previously subject to float imprecision).

BUG=skia:5288,chromium:598484
R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1974463002

Review-Url: https://codereview.chromium.org/1974463002
2 files changed