Cleanup SkImageGenerator, add missing fns to GrBackendTextureImageGenerator

No "real" API changes.
TBR=reed@google.com

Bug: skia:
Change-Id: I08c29f76806388394938f204f2a50b2fd6ea8942
Reviewed-on: https://skia-review.googlesource.com/16662
Reviewed-by: Brian Osman <brianosman@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
4 files changed