factor out flattening/unflattening of common fields from SkImageFilter

This is a precursor to changing SkImageFilters (and all effects) to unflatten via a factory instead of a constructor. In that world, each subclass of ImageFilter will need to control/initiate the unflattening of the common fields, so they can be extract and passed to their Factory.

R=senorblanco@google.com, robertphillips@google.com, mtklein@google.com, senorblanco@chromium.org

Author: reed@google.com

Review URL: https://codereview.chromium.org/395273002
2 files changed