Use GrVertexWriter in GrLatticeOp

Adds TriStripPoint helpers to replace usage of SetRectTriStrip

Bug: skia:
Change-Id: Iced3dd4474caafb0a3c6ef057e30e227dbee88e0
Reviewed-on: https://skia-review.googlesource.com/c/171004
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
2 files changed