Refactor pixel conversion: premul and unpremul

Just going for simpler and more code sharing.

BUG=skia:

Change-Id: I84c20cd4dbb6950f7b4d0bc659c4b3b5a2af201c
Reviewed-on: https://skia-review.googlesource.com/8287
Reviewed-by: Mike Klein <mtklein@chromium.org>
Commit-Queue: Matt Sarett <msarett@google.com>
5 files changed