provide safe exit for runaway intersections

Curve intersections with extreme numbers may
cause the intersection template code to loop
forever. Detect this by looking for
marking more spans gone than exist, and
return without any intersections found.

TBR=reed@google.com
BUG=643855
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2310113002

Review-Url: https://codereview.chromium.org/2310113002
2 files changed