Fix yet another convexicator issue

Fix yet another convexicator issue

This CL fixes the case where a bad initial vector (i.e., nearly zero) managed to short circuit all of the convexicator's logic. The initial bad vector would become the last vector and then never get displaced.

The history of this is:

https://codereview.chromium.org/298973004/
Switched the convexicator to not advance the last vector when the cross product wasn't significant

https://codereview.chromium.org/573763002/
Fixed a bug (crbug.com/412640) wherein a zero area path was being incorrectly categorized as convex b.c. opposite but equal vectors were not signaling concavity.

BUG=433683
Committed: https://skia.googlesource.com/skia/+/f52a063dfd7883bbaa58617416a48c2be724e36b

[robertphillips, 2014-11-17 19:26:46]

robertphillips@google.com changed reviewers:
+ bsalomon@google.com, caryclark@google.com

[caryclark, 2014-11-17 19:37:55]

lgtm

https://codereview.chromium.org/727283003/diff/1/src/core/SkPath.cpp
File src/core/SkPath.cpp (right):

https://codereview.chromium.org/727283003/diff/1/src/core/SkPath.cpp#newcode2245
src/core/SkPath.cpp:2245: if (!SkScalarNearlyZero(vec.lengthSqd(),
SK_ScalarNearlyZero*SK_ScalarNearlyZero)) {
looks like SK_ScalarNearlyZero*SK_ScalarNearlyZero is a number pulled out of a
hat (but so is SK_ScalarNearlyZero)

it would be neat to test what the empirical limit of vec.lengthSqd() really is
-- but that can wait for another day.

[robertphillips, 2014-11-17 20:02:48]

The CQ bit was checked by robertphillips@google.com

[commit-bot: I haz the power, 2014-11-17 20:03:55]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/727283003/1

[commit-bot: I haz the power, 2014-11-17 20:11:47]

Committed patchset #1 (id:1) as
https://skia.googlesource.com/skia/+/f52a063dfd7883bbaa58617416a48c2be724e36b