Expose the owning surface to a canvas?

Handy if you want to flush, or otherwise understand the underlying buffer(s).

Change-Id: I3d6610695c8603232192b26c687c6c74512165dd
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/254803
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
3 files changed