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>
5 files changed