Move Ganesh Skp texture code into GrImageUtils

This is a follow-up to http://review.skia.org/673717 which
makes SkPictureImageGenerator Ganesh-agnostic (and moves it
to src/image/ so it can be closer to its friends).

Change-Id: I61e0e8d5e026fd782da9651c4d90be51daee0734
Bug: skia:13983
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/678656
Commit-Queue: Kevin Lubick <kjlubick@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
15 files changed