Hoist calculations up to regenerate

After this transformation, I think much of the
code in regenerate is the same.

Change-Id: I6ae3668181c6184b085b2b9862d80db1a23de586
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/263203
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Mike Klein <mtklein@google.com>
2 files changed