Reland "drawVertices in GPU backend uses drawVustomMesh implementation."

This is a reland of 73657689b6cb53d20ee6a1d9f34bdb5679fd78c4

Original change's description:
> drawVertices in GPU backend uses drawVustomMesh implementation.
>
> This still allows batching across view matrix changes but does not batch
> across SkVertices that have different attributes.
>
> Bug: skia:12720
> Change-Id: I34b58e2ae3aa7330b1fe3280b9448197e2bb68b0
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/491047
> Reviewed-by: Greg Daniel <egdaniel@google.com>
> Commit-Queue: Brian Salomon <bsalomon@google.com>

Bug: skia:12720
Change-Id: I24230e2a284b98ce71ccf034ff71d903896f1da1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/492357
Reviewed-by: Greg Daniel <egdaniel@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
8 files changed