added caching of SkVertices

SkVertices can now be "volatile", meaning they should not be cached.
SkVertices is volatile by default if the argument is not given.

Pulled from reverted CL: https://skia-review.googlesource.com/c/skia/+/138596

Docs-Preview: https://skia.org/?cl=139545
Bug: skia:
Change-Id: I92cf832efe1c0aaa8f432eedde2678582dd2454e
Reviewed-on: https://skia-review.googlesource.com/139545
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Ruiqi Mao <ruiqimao@google.com>
9 files changed