Reland "generate vertex data in onPrepare"

This is a reland of a33a68071b63bbed85e9fadb47dd4ae8da8d519f

Original change's description:
> generate vertex data in onPrepare
>
> Switch to a compact encoding for vertex data
> that can be easily expanded in onPrepare.
>
> Bug: skia:10251
>
> Change-Id: I53893c94514a7ff3b4f33be444f4ec2002e63ec4
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/290297
> Commit-Queue: Herb Derby <herb@google.com>
> Reviewed-by: Robert Phillips <robertphillips@google.com>
> Reviewed-by: Ben Wagner <bungeman@google.com>

Bug: skia:10251
Change-Id: I483f0d48b475d8fdea2ea915e2f954d5aec3ac0d
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/292077
Reviewed-by: Herb Derby <herb@google.com>
Commit-Queue: Herb Derby <herb@google.com>
5 files changed