Fix SkRadialGradient center discontinuity

(AKA let's slow down radial gradients!)

Avoid discontinuities when r ~= 0.

BUG=skia:5052
R=reed@google.com,mtklein@google.com
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1753133003

Review URL: https://codereview.chromium.org/1753133003
1 file changed