This will allow a client to make an SkImage that "wraps" a gpu texture,
however the client does need to supply the actual gpu texture at Image
creation time. Instead it is retrieve at flush time via a callback.
Bug: skia:
Change-Id: I6267a55ab7102101a7bd80a6f547b6a870d2df08
Reviewed-on: https://skia-review.googlesource.com/109021
Commit-Queue: Greg Daniel <egdaniel@google.com>
Reviewed-by: Robert Phillips <robertphillips@google.com>
Reviewed-by: Cary Clark <caryclark@google.com>
