Fix is_convex() check for inset polys.

Change-Id: I1a8a24132b0cf9eb80dd586bf2f1b930adb7b582
Reviewed-on: https://skia-review.googlesource.com/10525
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
2 files changed