Make SkImage_Gpu be deferred (take 2)

This CL removes the GrTexture-based ctor forcing everyone to create deferred SkImage_Gpus.

relanding of: https://skia-review.googlesource.com/c/6680/ (Make SkImage_Gpu be deferred)

split out into:
https://skia-review.googlesource.com/c/9106/ (Remove atlas creation from GrResourceProvider)

Change-Id: I834ede430b9706cf9b675bdfdddf1c8c624c2f14
Reviewed-on: https://skia-review.googlesource.com/9965
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
17 files changed