add copytable method to typefaces

This allows some backends to efficiently retrieve a copy of the data.

Inspired by https://skia-review.googlesource.com/c/skia/+/229136

Change-Id: I9b0345333e30376a50a5dc7c9e993b847e2791c4
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/229384
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
Auto-Submit: Mike Reed <reed@google.com>
10 files changed