Add mask format to the SkGlyphDigest

Change-Id: Iab1be028b7923160256373536e354a6ce8d43159
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565646
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed