make float more primary API for paint (esp. for alpha)

Bug: skia:8773
Change-Id: I82b1f22f300eadc93f79a35a1638b7eb6376169a
Reviewed-on: https://skia-review.googlesource.com/c/193031
Commit-Queue: Mike Reed <reed@google.com>
Reviewed-by: Brian Osman <brianosman@google.com>
22 files changed