Never share pixels when we make a subset image

- mostly just affects Lazy images (raster was already always copying)
- issue with other factories that offer subsetting
   - (e.g. MakeFromGenerator)
   - can we remove those options, and require makeSubset() afterwards?
- greatly simplifies dealing with mipmaps

Landing this would obsolete
https://skia-review.googlesource.com/c/skia/+/305960

Related: https://skia-review.googlesource.com/c/skia/+/306136

Related: https://bugs.chromium.org/p/skia/issues/detail?id=10544

Change-Id: I074420543bd2fcd46ed1620bbf0eed00371ab018
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/305970
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Leon Scroggins <scroggo@google.com>
8 files changed