Fix color xform width bug when scaling/subsetting

This was not caught by the bots because we don't test
color correct modes with our many image decoding tests
(takes too long).

Adding a unit test.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2247743002

Review-Url: https://codereview.chromium.org/2247743002
5 files changed