Make SkColorFilter::appendStages() not fail.

This makes SkColorFilter::appendStages() first try onAppendStages(),
and if it's unimplemented or fails, fall back to filterSpan4f().

This also makes onAppendStages() private to try to ensure that
appendStages() is now its only caller, ensuring everyone goes
through this fallback path.

The fallback uses the color filter transformed into the dst colorspace
using our new SkColorSpaceXformer... that seem ok Matt?

Change-Id: I4751a6859596fa4f7e844e69ef0d986f005b52c7
Reviewed-on: https://skia-review.googlesource.com/16031
Reviewed-by: Mike Reed <reed@google.com>
Reviewed-by: Matt Sarett <msarett@google.com>
Commit-Queue: Mike Klein <mtklein@chromium.org>
4 files changed