Add SkDefaultXform as a catch-all to handle color conversions

I'd like to start optimizing the common case for color xforms,
but before doing that, I think it makes sense to have correct
code to support all xforms.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2038823002

Review-Url: https://codereview.chromium.org/2038823002
5 files changed