Convexity checker: Wait for significant x-product when walking edges

This effectively re-applies the logic from
https://codereview.chromium.org/298973004

The change to the unit test for 389050 is interesting.
The last convexity rewrite "fixed" that case by allowing
it to be detected as convex. In the process, it actually
broke the original bug, so that rendering of the HTML
failed. This CL causes us to give up and decide that the
path is concave, but we return to rendering correctly, so
that's a win.

The bug that initiated this (950508) is effectively the
exact same bug as 2235, which is why I haven't added a new
test case. The existing test case is much more concise than
the 100K data file needed for the new one.

Bugs: skia:2235 chromium:389050 chromium:950508
Change-Id: I0de65db8644f37e335c47e9d41c676b8e8b020fc
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/209164
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
2 files changed