Update SkPMFloat API a bit.

Instead of set(SkPMColor), add a constructor SkPMFloat(SkPMColor).
Replace setA(), setR(), etc. with a 4 float constructor.

And, promise to stick to SkPMColor order.

BUG=skia:

Review URL: https://codereview.chromium.org/977773002
7 files changed