Optional local matrix for SkShaders factories

Change-Id: Id913f89c699097ce53c9f8ef6deea264811a4179
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/226082
Commit-Queue: Florin Malita <fmalita@chromium.org>
Reviewed-by: Mike Reed <reed@google.com>
5 files changed