Use GrVertexWriter in GrSmallPathRenderer

Make TriStrip a templated rect, with a helper for SkRect.
Added writeQuadValue overload for GrQuad.

Bug: skia:
Change-Id: I2934e5dbb086c5dbe0cc4846176bab8ccc324cf6
Reviewed-on: https://skia-review.googlesource.com/c/171534
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed