Ensure the Gaussian tail of circle blurs goes to zero.

Ensure the Gaussian tail of circle blurs goes to zero.

Compare blurcircle GM masked so that values <255 are white and =255 are black:
https://x20web.corp.google.com/~benjaminwagner/review/1723383002/blurcircles-lt255-before.png
https://x20web.corp.google.com/~benjaminwagner/review/1723383002/blurcircles-lt255-after.png

This is a spinoff of https://codereview.chromium.org/1691403002, which was previously causing more "rectangles" in this GM.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1723383002

Review URL: https://codereview.chromium.org/1723383002