Add SkTypeface::openExistingStream.

Allow users to retrieve the stream from an SkTypeface, but only if that stream is inexpensive to create.

Bug: https://github.com/flutter/flutter/issues/97384

Change-Id: I9bab1a775273363500cf7482f1babeee75c5a3df
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/503349
Reviewed-by: Ben Wagner <bungeman@google.com>
Reviewed-by: Derek Sollenberger <djsollen@google.com>
Commit-Queue: Julia Lavrova <jlavrova@google.com>
5 files changed