make prepareGrGlyphs private

Change-Id: I0927b8a468188b358a87f362a528fe32cf9aec71
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/303577
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed