Add GrMakeCachedBitmapProxy

This is split out of https://skia-review.googlesource.com/c/7889/ (Remove SkSpecialImage's GrTexture-based ctors) to allow focusing on the caching/subsetting changes.

Change-Id: Ic2af7a0d03e88941ab5b6cdfcecbbdefe3578eda
Reviewed-on: https://skia-review.googlesource.com/8456
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Robert Phillips <robertphillips@google.com>
5 files changed