We already do this for cached SkSurface snapshot images. However,
sharing the SkPixelRef among any SkImage_Rasters with different
SkMipmaps interacts badly with SkImage texture caching and likely
other image caches.
Bug: skia:11983
Change-Id: I07bb7a93ffb79afb7f8268cd5159ee22f58da1e7
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/526456
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>