Use SkTransferFunctionBehavior in SkImageGenerator

This fixes SkColorSpaceXformCanvas gms that expect
non-linear premuls from the codec.

BUG=skia:

Change-Id: I5dc236d0cd760c23605a26e9c33ddb18955f9231
Reviewed-on: https://skia-review.googlesource.com/10164
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
7 files changed