Make fMaskFormate private

Change-Id: I20e652f2b6f9bf606b03c6dd4e346c3439ea8a0b
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/220876
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Herb Derby <herb@google.com>
10 files changed