GrTessellator: fix overlap outer boundary winding.

When two outer boundary edges are collinear, they are part of an
overlap region. As of 77169c8fd6d, we (correctly) leave these edges
connected, but we must also adjust the winding to magnitude 1 to
keep the winding rules correct.

Note: this adds a new test case to the concavepaths GM, affecting all platforms.

Bug: 863389

Change-Id: I7e3a06df537cd189101e7ad39a4815a78be8fbdd
Reviewed-on: https://skia-review.googlesource.com/141952
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
2 files changed