Identity unichar <-> glyph_id mapping for SkCustomTypeface

Would be neat to support explicit mappings, but this is sufficient for
what I'm looking at now (allows passing custom tf + "strings" through
the existing shaping pipeline).

Change-Id: I62a8a0c90cc9f6bf3ede82932a8b6a2a933521c1
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/290197
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed