Avoid unneeded picture shader color space transforms

Also update SkPictureImageFilter to use SkColorSpace::Equals instead of
direct comparison.

Change-Id: I434cd6d1afcb869d2a56601b72edf56d207d9868
Reviewed-on: https://skia-review.googlesource.com/23201
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
2 files changed