Switch SkImageGenerator over to generating GrTextureProxies

It does not seem irrational for generateTexture to always receive a valid GrContext. lockAsBitmap can do as it pleases.

This is split out of: https://skia-review.googlesource.com/c/8823/ (Remove GrFragmentProcessor-derived class' GrTexture-based ctors)

Change-Id: I8aebc813a8a3a7d694b7369c2c9810e2164fe16e
Reviewed-on: https://skia-review.googlesource.com/9191
Commit-Queue: Robert Phillips <robertphillips@google.com>
Reviewed-by: Brian Salomon <bsalomon@google.com>
8 files changed