Improve SkVerticesPriv ergonomics

Rather than two separate (partially overlapping) ways of accessing the
private portions of SkVertices, use a single privileged helper class
(similar to GrContextPriv).

Change-Id: I76b14b63088658ed8726719cce126577e5a52078
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/280601
Commit-Queue: Brian Osman <brianosman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
12 files changed