Sk4fLinearGradient fuzzer fixes

1) update in_range() to actually follow the documented rules regarding interval
   open/close endpoints

2) detect cases where the intervals provide negligible advance and fall back
   to using a color average

BUG=skia:5647
R=reed@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2472483002

Review-Url: https://codereview.chromium.org/2472483002
2 files changed