Fix Sk4fLinearGradient initial interval pinning

For repeat/mirror mode, fx tiling needs to ensure the value doesn't
collapse to the open interval value due to float arithmetic.

Clamp to the next lower representable value.

BUG=skia:5955
R=reed@google.com

GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=4784

Change-Id: I635394d15a80276e88c18a499f93f3047464f190
Reviewed-on: https://skia-review.googlesource.com/4784
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
1 file changed