add constexpr constructor to SkMatrix

This is an experiment in making SkMatrix less sharp.

Change-Id: Ie1881b0905b594aa1c444440fae5253970850e12
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/208340
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Klein <mtklein@google.com>
2 files changed