Add a per-triangle vertex advance to GrMiddleOutPolygonTriangulator

This will allow us to write the triangles with a 4-vertex stride and
combine them with cubic instances for indirect draw mode.

Change-Id: Ibe1b16ac093c2fd09a604f37e527c1c24369a64b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/289936
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Michael Ludwig <michaelludwig@google.com>
2 files changed