GrTessellator: fix edge winding merged to zero.

GrTessellator: fix edge winding merged to zero.

[cherry-pick of 531a48ed788c5fabfe21704286b54d7567f35469 to M68.]

Occasionally, the top edge of a split edge can be merged out of
existence (if it can't be mathematically distinguished from one of its
adjacent edges). In this case, we need to save its winding count
before it's merged so that the other half of the split doesn't also
get zero winding.

NOTE: this adds a new test case to the concavepaths GM.

No-Tree-Checks: true
No-Try: true
No-Presubmit: true
Bug: skia:7785
Change-Id: Iff8b0a2ed3065e57b737ff915ff436146987caf6
Reviewed-On: https://skia-review.googlesource.com/131223
Reviewed-By: Robert Phillips <robertphillips@google.com>
Commit-Queue: Stephen White <senorblanco@chromium.org>
Reviewed-on: https://skia-review.googlesource.com/137600