Support color xforms for kIndex8 pngs

This change started as: "Always use color xforms to
premultiply".  We need to be in a linear space to
premultiply correctly.

It became clear that we also need to support kIndex8
color xforms in order to make this change.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2246143002

Review-Url: https://codereview.chromium.org/2246143002