add helper to know when a colortype is always opaque

Now that surface creation is more picky about its imageinfo, we need to
allow clients to know when they should clean-up their alphatype (like
our own gm)

Bug: skia:
Change-Id: Ic110c75769e0154a8343d7e2160d3351f02cf48f
Reviewed-on: https://skia-review.googlesource.com/106320
Reviewed-by: Mike Reed <reed@google.com>
Commit-Queue: Mike Reed <reed@google.com>
3 files changed