Add alphaType() to SkImage

Keep isOpaque as a convenience method -- many places really only need to
know that for optimization purposes (SrcOver -> Src, etc...).

In all the places where we pull data back out or convert to another
object and need to supply an SkImageInfo, we can avoid losing information
about premulness.

BUG=skia:
GOLD_TRYBOT_URL= https://gold.skia.org/search?issue=2250663002

Review-Url: https://codereview.chromium.org/2250663002
11 files changed