extract strike-to-source scale from SkStrikeSpec

Making the strike-to-source scale part of the SkStrikeSpec needs
to be handled more carefully now that the "SkMatrix is always the
same" invariant is broken.

Start with MakeCanonicalized focusing mainly on SkFont.

Change-Id: I8e3ab6120751ce280fea03b87be200155d736130
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/467080
Reviewed-by: Ben Wagner <bungeman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
4 files changed