Add GrBackendTexture accessor to SkImage (take 2)

This makes accessing the GPU resource behind an SkImage a lot more typesafe. Additionally, the GrBackendObject is being deprecated so this is the path forward.

I split the controversial stuff off into https://skia-review.googlesource.com/c/skia/+/118575 (Add SkImage::setLayout call).

Change-Id: I297e72770e8fb360fac7c7cd74f050ae759ae133
Reviewed-on: https://skia-review.googlesource.com/118571
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
14 files changed