Make mask checking explicit in SkDrawableGlyphBuffer

This is in preparation for not passing SkGlyph* around
because it will not be available from the RemoteStrikes.

Change-Id: I9a7833c7a23726020aefdff41766952458c61f72
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/565636
Reviewed-by: Robert Phillips <robertphillips@google.com>
Commit-Queue: Herb Derby <herb@google.com>
3 files changed