Isolate fGlyphs in SubRun

Add glyphCount to return the number of glyphs and
grGlyph to return a grGlyph from fGlyphs.

Bug: skia:10251

Change-Id: I15f77a032b9eb9aa736ccd810900f2425fc4c059
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/290296
Commit-Queue: Herb Derby <herb@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
4 files changed