Convert GrVertexWriter to a "stream syntax" API

This helps remove all Ganesh dependencies from GrVertexWriter so it can
be shared with Graphite.

Bug: skia:12466
Change-Id: I1ede1cfc48d501cb1e028bca1e8590e6c92f4e4d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/462316
Commit-Queue: Chris Dalton <csmartdalton@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
25 files changed