Add a paint filter utility canvas (SkPaintFilterCanvas)

Introduce a paint filter proxy base class as a SkDrawFilter replacement,
and convert SkDebugCanvas to use the new approach.

BUG=skia:3587
R=reed@google.com,mtklein@google.com,robertphillips@google.com,tomhudson@google.com

Review URL: https://codereview.chromium.org/1032173002
5 files changed