Add new SkImageGenerator::getPixels() API, deprecate the old

This is fairly aggressive in that it will break any client
that is currently using SkImageGenerator with kIndex8.
I'm guessing that we don't have any clients doing that.

Bug: skia:6620
Change-Id: Ifd16f5232bb3a9f759c225315c57492d917ed9ca
Reviewed-on: https://skia-review.googlesource.com/16601
Commit-Queue: Matt Sarett <msarett@google.com>
Reviewed-by: Mike Reed <reed@google.com>
16 files changed