consolidate common vertex filling code

Change-Id: I41756f8e01a2ff0782612cf08dca27188263276e
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/305963
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Ben Wagner <bungeman@google.com>
1 file changed