Add svg path arcto

The arcto() used by SVG in Chrome and Android is ported here, using conics
instead of cubics. The logic is a direct transposition of the WebKit code.

The attached GM includes SVG that draws the same as Skia.

R=reed@google.com
BUG=skia:3959
GOLD_TRYBOT_URL= https://gold.skia.org/search2?unt=true&query=source_type%3Dgm&master=false&issue=1613303002

Review URL: https://codereview.chromium.org/1613303002
4 files changed