Fix missing triangle when miter point omitted in GrAAConvexTessellator.

This issue is seen in the existing GPU results for GM "strokes_round".

Detected while investigating the below bug. However, the real bug there
caused by bad tessellations appears to be coming from
GrAAConvexPathRenderer.

BUG=chromium:851637

Change-Id: If4e653d10823d53f68bb9bf6bb77875cf391426f
Reviewed-on: https://skia-review.googlesource.com/151368
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
1 file changed