Split SkCrossContextImageData into Image and BackendTexture subclasses

This is needed for a follow up CL where we switch from using
GrBackendTextureDesc to GrBackendTexture.

Bug: skia:
Change-Id: Id12d3357af1cc7f30c7f9196e614089779dc0715
Reviewed-on: https://skia-review.googlesource.com/13588
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Greg Daniel <egdaniel@google.com>
4 files changed