fix arcto exception handling

To match SVG behavior,
draw a line when either radius is zero,
or when the end point equals the start point.

Update the GM to include these tests, and
take the scale used to test it on a highres display.

R=fmalita@chromium.org
BUG=skia:4849
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1627423002

Review URL: https://codereview.chromium.org/1627423002
2 files changed