Move new WriteVertexData utility to a GrVertexWriter helper struct

Encapsulates the pointer storage and advancement, shortens usage.

Bug: skia:
Change-Id: Id06cd273ed638395d1fd53b3b66f87d4d33a4f3b
Reviewed-on: https://skia-review.googlesource.com/c/170727
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
6 files changed