Make fMaskFormat private

Change-Id: I7543a29e9b8a6b5e372f940d5662e66d9ac2169b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/293249
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
2 files changed